Re: Raven Feast, the formula is right in the book. 'The exchange rate is gems = sqrt(corpses)/3.' (DomIII Rulebook, p.208). Since it is 4 gems, the 'normal' conversion rate from Air
eath is 4:1. Substituting that into the formula, you can solve for how many corpses you need to generate how many gems. Here is a quick table.
Gem Corpses
1 9
2 36
3 81
4 144
5 225
6 324
etc.
Note this means that if there are 36+ corpses in a province, you will receive more gems than through alchemy. The really interesting part is that at 144+ corpses, you get at least a 1:1 ratio.
This is in addition to the fact that it eliminates all corpses in the territory, which can be useful in its own right.
The nations I can see this primarily useful for are EA Caelum (definitely can cast it, have Death mages with no Death income, IIRC), EA Ermor (would need pretender, but same dilemma), and, ironically, LA Ermor. Again, needs a Pretender, but their entire engine runs on Death gems, and they have the corpses to burn (almost literally).
Fenris: In regard to expansion, you generally want to take one province _or more_ starting from the third turn.