How much I am willing to pay for a bookmark (or anything at all) depends on four factors, listed by order of importance:

1. The artist.
2. The art style.
3. The quality of the merchandise.
4. The publisher/co. that made it (that is, whether the product is copyrighted).

Obviously I'm willing to pay good money for a bookmark made by a professional mangaka. But I can also be persuaded to spend the same amount of money on a fan artists' work (both fanart and original art) if I like their art style. For example, I never hesitate to buy P.L. Nunn's doujinshi, prints, or other merchandises that I like. The price is not a factor there, though I have to say she charges very fairly.

So I can't really give you a price until I've seen the product.