Sell value to make up for nerfs. Current and Future.

I think they should make the sell value for all HP and ISO of a character equal 67% (maybe 75%) of how much you put into it. You SHOULDN'T be able to get 100% back because you did enjoy the pre-nerfed form of the character and used them to your advantage during that time.

For example, 5/5 rags but you only put in 5000 HP for 4 skill levels. You'd get 3333 HP back (67%).
You'd get 67% of ALL ISO you put into leveling him.

This way, when characters get nerfed, you can get something back for putting your time into it while the developers don't have to worry about issuing refunds.