Well from what I've seen, Brickheadz tend to go for 5 to 10 bucks. Given that these are custom and made independently, you could probably go for 12 or 15. 20 I think would be a big stretch, given the general size of a brickhead. Though you also have to take into account how much it costs you per brickhead, so you end up making a profit instead of just breaking even.
All things considered, I think this is a pretty good idea, though you might want to try to advertise yourself in a few more places. Go out and try to market through other social media platforms to maximize your audience.