15% sounds rather low, and it really depends how much their retail price is. if they really sell a 1000 a day for example, it might be ok. but art usually sells now and then. by "rights" what do they mean exactly? are these exclusive rights?
if you can sell your work on other things to other people. and you contract outlines exactly what they can do with it, then its an extra source of income. if they only want to give you that much (and make sure its off of retail, because one guy tried conning me for the whole sale price which was in cents), and they want it exclusively. i would say no.
---Mike Savad
http://www.MikeSavad.com