I'm the same way. I want to buy a wallet, but its so close to the price of a purse. If I were you I would go the cosmetic bag. You would probably use it more than the purse and you'd be adding to your SLG.
Cosmetic case! I was so hesitant at first because I didn't want to pay 300 bucks for a cosmetic case, especially since I don't like to carry makeup around with me. But it is one of my favorite LV purchases. I use it every single day and it's great for holding all the little tid bits in my bag. Makes switching bags so much easier.
I say save for a purse. There are a bunch of cute companies that make cosmetic cases that will compliment your LV accessories. I like quirky and cute cases like ones from TB or KS or especially henri bendel. But if you have money to throw around and don't have anything burning on your wishlist and want to buy something then go for it.