In my years of gaming, I have noticed that abilities with no energy cost whatsoever are immensely more useful than those with even a minor cost. Take a super with a laser eye-beam. If he gets into a fight, there's no reason he can't use it 20 or 30 times in the span of a combat. Add even a 1 FP cost to it, and suddenly he can't use it more than 7-8 times without seriously imparing his ability to fight -- and that's if nothing else fatigues him (or already has)!

Note the -40% cap, however, to prevent this from becoming a free ride. Otherwise you find the munchkins buying their FP up and then adding "Costs 40 FP, -200%" to bring down the cost of their 500-point ability with a ton of enhancements. Also note that there is no "Costs 2 FP per 10 seconds" limitation. This rewrite gives a fair point break while preventing the worst instances of abuse.

Optional Complications: "Costs Hit Points" is worth 1.5 times as much -- hit points are harder to recover and more dangerous to lose. If you have any abilities that allow you to recover at least 1 FP or HP (as appropriate) per second, this limitation is worth 1/5 as much. If you can recover faster than that (e.g., Extreme Regeneration), it's worth 1/10 as much. The fact that you have to pay the energy cost is still a minor limitation, on about the same level as a Nuisance Effect.