On the other hand you have to keep the game competitive for new teams.
i would say, salary increase around 12-30k for one Rebounding Skill, ain't encessary and not in balance to the extra effort the player brings at the top centers ;) If you reduce that, new teams still have a chanche to get to the top. Especially when a skillup betwen 7-8 is lot more usefull then one betwen 16 and 17(but salary wise it is totally different).
They must not be cheap, but salarys above 200k are hard to pay even for top teams, so that they won't run with four player of that kind ;) Maybe you could afford one of then, and for that making
small compromise for the rest of the team.