I have not seen this in a lot of video game RPG except Final Fantasy 1. In that game, magic users need to buy their spells to learn them and they can only learn 3 out of 4 spells giving them different spell configurations.
Non-Magic users generally have to spend a lot of money on weapons, armors, expandables and other stuff. On the other hand, Magic Users have limited equipment they can buy, so the spendings of magic using character is generally lower than other classes.
So to even things out, paying to learn spells could be a way to make sure magic users requires as much money than non-magic using classes. This seems essential if each character has a separate pool of money. If the gold is pooled for the party, there will be less reasons to even the spendings of different classes.
Another interesting feature in Final Fantasy 1 is preventing the player to buy all spells for his character. So he must make a choose which spell to acquire. The problem is that you could get a spell that ends up beign less useful than you though, than means you are now stuck with it. An alternative could be that you equip spells, when there there is spells that you don't like, you can unequip them and replace them with other spells.
Incorrect, the higher in rank the character, the greater to odds to get attacked. This is why your fighter in front line always get most of the hits.
Currently, I made spells works like attacks and having a chance to fail (Like in DnD4th ed), but there are much less defenses available. But the problem is that you pay MP to cast a spell, you do not pay anything for regular attacks. Since you pay for it, I though that having a spell that could fail makes the cost too high. There needs to be something else to compensate, like more powerful outcome than a simple attack. Or maybe less natural defense than regular attacks.