Screen printed booklplate

I've managed to produce a few of these, even though I'm not so hot at registration and at least half of them were too crooked to use, but at least the colours came out well. It's hard to see in the photo but the black is more of a dark bluish green. The print is modified from the bookplate stamp I made my sister last year, she's a total bibliophile and stamps all her books before lending them out.
I'm thinking of selling these in sets of two or three, but I'm not too sure how much to charge for them, any suggestions?