Ok
at the risk of being the devil's advocate here, you are asking for exclusive rights to someone's artwork and offering to accept only the cheapest quote
Is that not exploiting unknown artists who want to get their work out and about?
Whilst I understand that you do not want to see your chosen images plastered across a whole host of rival t-shirts and goodies, the rights do usually remain with the creator of the work, and to get an artist to give these up, usually requires a fair price. Or at the very least, there should be a limited license to the rights (ie for a set period of time or for distribution only within a limited area).
How about a profit share system? That would be a little fairer on the artist, and mean that you don't have any massive set up costs. Just an idea