That is not an unreasonable price to pay for a stellar design. Too many people have the, "everything should cost $5" mentality. The only things that actually cost $5 are things that someone can create at the push of a button. Inexpensive designs look terrible.
To make money on Teespring, though, you really need to have someone on your team who can do the designs for free. Then you can do lots of campaigns and your only cost is FB advertising.
You also have to be very targeted, as someone else said. And your designs have to be very, very original. It's not just the artwork that matters, but the composition.