I've found when playing aliens that sometimes there'll be one or two people on one team who are doing realy well and killing heaps of enemies (yay goon!), to the point where they have 9 evolve points just sitting there.
I think it should be possible for players to buy/evolve higher than the stage their in for twice the cost. So, if I'm in S1, and I'm a dragoon with 9 points. It'd be 1 point to go dragoon -> adv dragoon.
But thats up in S3, 2 stages higher, so make it cost (for example) 1 * 2 * 3 = 6. So in S1, it'd be 6 points to go dragoon -> advanced dragoon.
ie. <point> * <S2> * <S3>
If you were in S2, and wanted to go goon -> adv goon, that'd cost 3 points.
For balance issues, perhaps change the S3 extra multiplier to 4. Then S1 goon -> adv goon would be 8 points.
The same applies to human weapon costs.