If someone wishes to buy legal rights to your design, it obviously has some value for them. How much you wish to charge them, however, is completely up to you. It kind of comes down to how much your client feels the design is worth to them.
The best example of how you might want to charge them comes from the world of stock photography. Most stock photo houses charge by the USAGE and the AUDIENCE size. So, for example, if a designer wants to use a photo ONE time for ONE postcard design for a mailing to 500 people, that will cost a lot less than if the designer wants to use the same photo for UNLIMITED use all over the world.
If your client just wants to reprint your t-shirt, you should charge them a flat fee to do so. But, if they want the right to print your design anywhere they see fit - on their trucks, on their building, on the website, on sales flyers, etc, then they should have to pay you extra for that privilege.
It's hard for me to put a price on your design without seeing it, but a minimum of $500 dollars for an original design would not be beyond reason. I've worked with companies that have spent thousands.
I hope this helps, Holly.
I know this will sound clichè, but go to Forever 21! The clothes are beautiful and at a very fair price! I got a cute dress for my eighth grade dance for 14 dollars! I don't own anything from Aero, AE, or Hollister, but what I do is look up the seasons current trends and buy super cheap clothes from thrift stores, yard sales, etc. It doesn't matter where you get it, no one will know, of course unless it's like Hollister or whatever. Good luck!