Personally, I feel $1 (or the classic "$.99" tag) would be perfectly reasonable. HOWEVER.. SS would need some way to put in the ability for a theme to be installed and tried out for.. let's say 2-3 days before requiring a payment for a given theme. I'd be really pissed if I had to pay $1 for every theme that worked better in the developer's screenshot than it does in working practice. I can think of more than a few themes that fit that description - as I'm sure all of you could.