Personally I think it costs way too much, and with the ways around the cost like BBB it's actually just a silly restriction.
I remember way back before 3e came out and a lot of how sorcerers worked was described. On the wizards board I asked, well why would anyone play a wizard then. The respose from one of the designers was because a wizard could easily know all the spells in the game.
So sometimes I wonder how much the cost was playtested. Was this one fo the things added in at the end. Something like a playtester asked is there a cost for scribing spells into your spellbook or is it free. And they said, hmmm I guess there should be a cost, lets go with 200gp a spell level that seem fair. But there wasn't time for the extensive playtesting to show exactly how expensive that really is.
If you just want 3 additional spells per spell level at level 10, you have 1/2 the money to spend as other characters. That puts you for 2nd, and 3rd level spells with not many more known than the sor, but a decent number more known of 4th and a lot more known of 1st and 5th. But that sorcerer can double the amount of equipment you carry, those little things in such an equipment heavy game that really decide whether you live or die, and usually save your life a lot more often than a few spells in your spellbook will. But then quickly after level 10, the money for the PCs jumps up dramatically, and the amount spent on the spellbooks for the wiz is just a small fraction of his total income. So were they just trying to balance things with the sor from levels 1-10 and after that just wanted to let the wiz run wild.
The end result is the limit only limits the wiz until level 5-7(or level 11ish if they don't get a BBB) until the wiz makes himself a BBB and then starts learning spells left and right. So either it was meant as a big balancing factor and they just failed miserably at it, or they just added a relatively arbitrary cost for some kind of internal game logic or something and again probably failed miserably.