I hate to say it, but if you're going to buy production rights off an artist be prepared to pay a neat sum. At least, in the professional world and if you want the artist to treat you professionally then the customer should do likewise.
My advice to artists is to clearly mention your policy on your price list, do you reserve the right to make prints of commissions or include it in portfolios? Do you intend to use that right? Or will you never sell prints etc. of paid commissions? Or will you accept additional payment to not make prints?
My advice to customers is, read the artists' terms and conditions. If you're not clear on something, ask them about it!
And if you don't like an artist's "terms of service", don't commission them.
Mind I'm speaking in general and not in relation to previous posts.