Hmm, after some experimenting, I've found these Elerium frequencies:
In a Medium galaxy, Testing with
NebulaeFreq / Resfreq:
- Abundant/Abundant: 21, 26, 24
- Abundant/Common: 10, 9, 9
- Abundant/Occasional: 4, 6, 5
- Common/Abundant: 22, 15, 19
- Occasional/Abundant: 17, 12, 16
- Common / Common: 8, 5, 10
- Occasional / Occasional: 6, 3, 5
So, we see the Abundance characteristic makes the biggest difference in how many Elerium resources there are, but is modulated by the number of nubulae.
This is a map with Rare asteroids, Rare black holes, Rare nebulae, and Abundant resources:
As you can see, not all resources are created equally 
Durantium is so abundant, because *at* Abundant, every asteroid belt has a 1.0 chance of containing Durantium. So even though the belts are small, they all have it. The 'Rare' setting for asteroids gives a range of 1-2 asteroids per belt (and all stars have at least 1 asteroid belt), so there is basically a guarantee of at least one Durantium roid at every star. Black Holes and Nebulae work independent of actual star count, so there's no minimum there. That's probably why we saw some maps with none at all.
Here are the same Rare/Rare/Rare settings, now with Uncommon resources:
Again, there are lots of Durantium resources (~30), because per asteroid belt there's still a 0.4 chance of Durantium. But no Black Holes to provide Antimatter, nor any Elerium nebulae at all.
Another thing I found from looking through the resource code is that from Uncommon through to Abundant, there is no difference in Antimatter per Black Hole, each setting has identical chances for 1, 2 or 3 antimatter (50%/40%/10%).
Here's a game set to None resources, but Abundant black holes and asteroids:
As you can see, there is still antimatter on the map. This is because MapSetUpDefs rates None resources as 0 for all other resources, but gives a 30%/50%/20% chance of 0, 1, or 2 anti-matter per BH respectively.
I tried this a few more times, with Rare Nebulae, and then with Abundant Nebulae.
Anti-matter with None Resources, Abundant Black Holes, Abundant Asteroids, Rare Nebulae:
9, 6, 4, 4, 8
Anti-matter with None Resources, Abundant Black Holes, Abundant Asteroids, Abundant Nebulae:
3, 3, 2, 4, 5
So it appears that Nebula count may be interfering with Black Hole (and consequently Anti-matter) generation. The reverse may also be true - possibly because it seems like nebulae and black holes appear in empty space between stars, and may 'fill up' that same area cumulatively.
In conclusion:
Number of Antimatter resources is mostly determined by Black Hole frequency, and is almost completely independent of Resource frequency setting. If you want antimatter in your game in any meaningful quantity, you go with Abundant black holes (test with abundant/abundant gave me ~10 AM for every ~15-20 Elerium and ~45-60 Durantium).
Number of Durantium resources is almost entirely and linearly determined by Resources frequency, has a very high minimum independent of the asteroid frequency.
Number of Elerium resources is primarily determined by Resource frequency. It is not really possible to significantly increase how much Elerium is available without increasing Durantium by a lot too. If you want a lot of Elerium, make sure you have Abundant Nebulae, and Abundant or Common resource frequency.
