Rush buy may be "inefficient", but there are certainly times when it's nice to have on hand.  I personally like to have a lot of cash on hand for either emergencies, ship upgrades, or diplomatic uses.

It may not be everyone's cup of tea, but I find it invaluable.

Indeed rushing has its advantages, if your playing with the ai on a low level you can get away with no rushing but it can still be helpful at time. Late game maintenance costs will also cripple you if you have completely ignored economic buildings. Maintaining a huge fleet gets expensive.

Generally though you don't needed as many economic buildings as production and research.

If they made it cheaper to upgrade ships by a big factor, then money would actually matter more. Right now there's no chance I'll blow 2k on upgrading a ship that I'm as likely to lose as to keep the next turn. The opportunity cost is way too high. Right now I treat ships as built-to-die because they won't remain technologically valid very long, you can't repair them, you can't upgrade them, you can build one each turn, and you don't really pay much for them (no credit cost, no limit at all outside of production which is an ever-available resource. You don't run out of metals. You don't run out of trained and eager to serve young men and women to have slaughtered at the front lines). 

Let it cost 200 credits to upgrade a ship to the next generation, and spending 2000k to upgrade 10 ships to a new shield and new torpedoes will make me put a higher value on that cash. Let me pay a little sum to repair a ship at a shipyard and I'll need to keep that cash flow positive. It would also make me care more about my ships and maybe start using unique resources on ships rather than planets all the time.

That would be my preference as well.  I've never liked that if you pay X you magically get an upgraded ship anywhere within your civ's borders.  Ship upgrades should never happen farther then 3-5 tiles away from a shipyard / dry dock.  Which would also give the developers room to cut the upgrade costs a good bit.  In similar games, you always had to get your ship back to a planet with a shipyard before it could be upgraded.  Maybe for refits you have to build a dedicated starbase within 2 tiles of a dry dock with a "refitting ring" on it.

Rush buy prices in 1.0.2 are about right in my view for colony improvements.  Maybe need balanced slightly, but I don't find them hugely over/under priced for the things that I tend to rush-buy (factories for a brand new colony).

I would love to see upgrading ships cost 50% less if done at a shipyard you own.

If people want a specific case for upgrading ships, it's Survey Ships.  I like to get a fleet of survey ships up and running ASAP.  But since I get the vast majority of my survey fleet long before I have enough attack/def to ward off pirate guarded anomalies, having cash on hand to upgrade those ships becomes VERY useful since I don't want to throw away the extra range, move, or scan that those survey ships might have gotten by destroying them. 

And even if upgrading survey ships to attack/def capable ones doesn't enter into the equation, just upgrading survey ships proper is a good idea since, as I said, I don't like throwing away the accumulated bonuses they might have received.

I've managed to make a loss once, but truth be told you can use one level 10 planet set to 80% econ, 20% manufacturing and that'll fund a 40-50 planet empire comfortably. Money is so easy to come by presently, and so limited in what you can do with it, that it's barely worth having in the game.

I think you've had a bit of a logic fail there. If one or two medium economy words can comfortably support 50+ production/research planets by mid-game, then they can support 20-30 even more easily. If you're struggling to get cash, then try specializing a couple of decent-sized worlds in Econ rather than using the empire-wide wheel - you'll be drowning in cash in no time. It's very, very hard to find yourself running a budget deficit past turn 50 unless you rely completely on empire-wide management tools.

Even if holding 50 worlds means you've already won, the fact that you've managed to acquire 50 without needing to ever set up more than a couple of bank worlds means money is too easy to come by.

It doesn't need to be much harder to get - I think maintenance could be increased globally by a couple of hundred percent and still not cause anyone any great difficulties.

I play MP and the Econ wheel & slider settings _still_ aren't being remembered between saves, which pretty much means you must rely on Imperial Econ settings for all worlds unless you enjoy remembering/setting the planet Econ every time you restore a game.  (Protip: Don't.  It gets old very quickly indeed.)  That having been said, even when relying entirely on all planets set to Imperial Econ it's still a fairly trivial matter to turn a profit after 50 turns.

I'd also like to point out that money is very useful in trading.

Especially when I don't want to trade my techs or resources to potential opponents, yet would like to obtain some juicy tech they have.

Buying various treaty agreements also works wonders to overcome negative diplomatic reactions.

In trade, money is the great lever. The more I have, the more powerful I can get.

Exactly right.  

I rush buy very rarely, but I like to trade money for techs and resources where possible.  Money and open borders/non-aggression for techs.   Sometimes buy a treaty, can be a  life saver.  (Since at higher difficulties you are usually behind at the start for quite a while.)    And a bit of a stash for a rainy day.   Rainy days don't happen often in space, but you can never be too careful.

I take sharp issue with anyone who says money isn't worth much in the game as it stands.  Perhaps some tweaks might be useful, perhaps a bit more maintenance, I dunno.

But I love my colony with eleventy-seven markets surrounding a financial capital.  Yummy.

If anyone has any worthless cash they want to pass along, our Brinks Ship will visit you for FREE.

I wouldn't say it's worthless. I'd say it's too easy to get, which is even more true considering that you can use it to complete the tech tree without ever trading away a single tech - in fact, it SHOULD be nearly worthless in trade considering how easy it is to come by. But the AI over-values it, leading them to trade away half their techs for the cost of rush-buying one colony ship. They take no account of the fact that, if they're swimming in cash, it might not be worth selling their techs to gain a relatively meaningless amount.

It needs a rebalance; rush-buy cost multipliers should be reduced considerably (and should scale with production already committed), upgrade costs should be nerfed through the floor (since currently there's no good reason to ever upgrade any ships - you can just build new ships near-instantly using hyper-industrial shipyards) and maintenance should be tripled or more so that building a big war chest actually requires some level of effort, rather than happening more or less automatically as long as any one planet is dedicated to banking.

I should not be able to become the financial capital of the universe when 90% of my planets have no financial buildings. You can't lead the universe in research without making 50%+ research worlds; you can't be a manufacturing colossus without making most of your planets industrial. So why can you have as much money as you could ever need from just one or two worlds?