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.)
Intermediate level front-end web developers are costly due to high demand and the work is arguably more difficult. I agree the $150 might be great for someone offshore though.
In any case I like the idea behind the $15 blind design since designers tend to suffer from "I like it but..." with indefinite tweaks and last minute changes. I wouldn't be surprised if customers end up going through several iterations...which would still be inexpensive in this case.
There's a class of customer that won't even hit the "rent a coder" type sites for work, but will instead pay significantly more to someone with a full, proper site and a reputation.
The real problem here is attaining that "full, proper site and a reputation." Getting those two elements in place could take years of work, some of it completely unpaid. I suspect the OP doesn't have either of those, and is using this as a way to build up some kind of portfolio (and certainly a lot of online visibility, which can be surprisingly hard to come by) as rapidly as possible while still pulling in some income.
Not that he needs to use any of the $15 work directly in his portfolio -- but if he hashes out lots of designs in a creative way, he's going to end up with some obvious winners that he can polish up.
