I think it's tied to your influence.  You receive a fixed amount per hex you control, I think...

Lright, that is something to go on.  But by god I'd like to see one area of the game where every bonus applies to net, such that a planet with a 50% building and a 35% building and a 25% planetary bonus would calculate: base * 1.5 * 1.35 * 1.25

/FutureIdea...

Meantime, until there is a way to sway tourism either to this planet or that one, tourism buildings it seems are all but worthless, and likewise the tourism tech branch

edit- let me clarify what I mean: unless there's a particular means to draw tourism away from a (for example) science world and direct it toward a general planet, where it can then be exploited with improvements, the buildings will remain impossible wastes of precious, precious tile space.

They increase the tourism income of the planet, not, to my knowledge, the planet's share of the base amount (which would actually be worse; if the Port of Call make a planet count as 1.1 planets for the purposes of determining base tourism income, it decreases the tourism income on all of your other worlds and doesn't increase the planet's total tourism income by as much as it would have if it had just increased the income generated by the share by 10%, and the net effect of adding a Port of Call to all worlds is similar to not having Ports of Call on any worlds, except that you've lost a tile in the process, and will be identical to everyone not having any Ports of Call if all of your competitors also placed Ports of Call on all of their worlds). The issue is that the base tourism income is often something like 2-3 credits per turn, and the tourism buildings might give you +10-50% + 5% per level (which is usually no more than 3, and typically would end up at zero), which works out to 0.2-1.5 credits of income plus up to perhaps another 0.5 credits from level bonuses on the tourism structure. A basic market increases your money multiplier by 0.25 + 0.05 per level on the market + 0.05 per adjacent tile containing a market, and so on a planet with a base production of 10 (5 population + colony capital) is worth up to 2.5 credits + up to another 3 credits from level bonuses on the market you have, with a maintenance cost of 0.25 credits. Since the level bonuses granted to adjacent income structures are the same regardless of whether you placed a tourism structure or a market, we'll ignore that for the comparison.

So, on a world with merely 10 production, you're looking at a fully-upgraded tourism structure that can give you maybe 2 credits a turn, or a basic market that can give you 5.5 * [savings fraction] - 0.25 credits a turn. Tourism structures just aren't a great deal. While you could perhaps make a case for building these on factory and lab worlds to help offset maintenance costs, these are not likely to be worth building on market worlds.

Currently, you should grab the Tourism tech itself fairly soon and then completely ignore everything else tourism related.Never build any buildings, don't research the other techs unless they're a means to get something else etc.

I'd suggest that, if SD really want Tourism to be viable, they reduce basic tourism even further and then give significant flat bonuses to tourism buildings - or tie them into global morale, possibly as a counter to LEP.