Lvl caps are 20/30/40/50
That also means that the exp curve for all the rarities are the same; the difference is where they end.
So then, why would you not want to throw 3145 exp at an SSR to get it to lv 30 first, then get the last 20 levels later? You'd still have 10,200 exp to go, but gem costs become more troublesome. The cost to enhance is based on the level of the base accessory and how many materials you use up; what exactly you consume to give it exp has zero bearing on it. It's level * # of materials * 100. So to enhance a lv 30, it's 30 * # of materials * 100 gems. Enhancing is basically a tradeoff between gems and higher rarity accessories; being more efficient with one requires more of the other.