There isn't a cost. Max level is often mostly down to background fluff. In short its down to the Narrator to balance it. Without a very good reason this will mosttly hovver at 5 or 6.
PC balance tends to come from the fact that a starting player is unlikely to be able to max out.
If you are looking for a species built to anything highr than 5 or 6 you may find that it gets arbitarily reduced by a narrator.
DanG/Darth Gurden
The Voice of Reason and Sith Lord
“Putting the FUNK! back into Dysfunctional!”
Coming soon. The USS Ganymede NCC-80107
"Ad astrae per scientia" (To the stars through knowledge)