As with most purchases, promotional logo items are priced by the quantity you buy. The more you buy, the more the price goes down. For example, 100 t-shirts could cost you as much as $20 apiece with printing, but 1,000 t-shirts might only cost you $5 apiece.
Many vendors will charge a set-up fee to set up and print your logo or message. Find out about any of these extra fees up front before you order your product. Also, look for the best quality you can afford. It won't pay if your product is shoddy and ends up in the trash.