Hello,
I'm an artist/graphic designer. My artwork is very valuable to me. I will want to maintain as much control of each design as I can. When I design a logo or illustrate a picture for someone I sell them only the usage of that image. I maintain the creative rights to the art/design. So if I were asked to draw a picture for someone I would ask what the artwork is going to be used for and how often it will be used. I would then make a contract for the usage of that to be signed by the client. This contract would state something like, "One time usage for 1000 t-shirts." The contract could be for a period of time as well, it just depends, each contract is different based on the client. If you wanted to purchase the complete rights to a design or illustration, meaning unlimited use for an unlimited amount of time, then I'm going to ask for a lot of reimbursement, because I will be forfeiting any possible future income of that design or art. Keep in mind, what if the client's business goes under in a year, can I resell the art to someone else? Maybe the client's business becomes very successful and they sell the company including the now intangible asset (my art/design) for millions of dollars. If I don't have a contract stating what my part in the ownership of that is then I get nothing.
Something to consider; if your designer hasn't already talked to you about this before the project has even begun then you may want to consider their experience. I would have this agreement/contract drawn up and signed before I even starting making anything. Working with someone inexperienced may be an advantage for you to get a lower rate, but it also might be a negative reflection of the quality of work you're getting. It might also save you some time and money down the road by avoiding lawyers, copyright laws, and cease and desist orders.
Good luck
David Lewis
Noble Soul Design