Money is a good gift, or pretty much anything that your daughter would like is appropriate. A couple of ideas: jewelry (whatever fits your price range), a "nice" book like a special edition of an author she might like (we got my niece a leather bound version of Lord of the Rings, and her brother the complete Far Side collection, plus some cash). I've bought small mp3 players for bat mitzvah gifts, which have gone over well in the past, but most kids seem to have them by now. I got my next door neighbor's daughter a point and shoot digital camera. A couple of years earlier we got her older sister an artsy necklace with matching earrings. (She had recently had her ears pierced, and she liked them so much she actually wore them for the bat mitzvah ceremony itself!)
We like to give something a little longer-lasting than the typical birthday gift, but anything in your price range is fine. For people we don't know so well, we usually spend between $25-50, for closer friends and relatives we'll spend more.
Hope that helps!