That's one of the troubles with these print on demand services- they're not cheap. How much would you consider a reasonable price? I'll search around and see if I can find a better deal somewhere.
Edit- Rereading the above, I sound a bit pushy. Sorry. I'm not trying to force mugs on anyone.
Personally, I think $10 would be good for a mug, but that doesn't seem to be possible. It might be possible if I were to pre-print a large number of mugs instead of using print-on-demand, but that would require me to buy a large number of mugs up front and hope to sell enough to break even. I'll do some more research on that.