Somebody could offer it sans HTML/CSS, charge $150 and still stay busy IMO. My basic mate's rates fee for that sort of job is $1000-1500 for a PSD.
That said, $15-20 is comfortably in that "If it doesn't work out, who cares" bracket. The problem will be that for many people, time is far more crucial than the cost. If I need a turnaround of 3-4 days, and the queue is 7+ days, it won't matter if it's $15 or 15 cents.
Have a good portfolio and gradually build up awareness. If you offer this service, link to a portfolio in your profile.
Don't compete on the same turf as elance types. I've used PSD2XHTML type services for $150-250 before and the main reasons I stopped was that the product wasn't really done to our house style and they couldn't hit deadlines. (Now I have staff that do these jobs for me.)
That said, $15-20 is comfortably in that "If it doesn't work out, who cares" bracket. The problem will be that for many people, time is far more crucial than the cost. If I need a turnaround of 3-4 days, and the queue is 7+ days, it won't matter if it's $15 or 15 cents.