The most affordable mainstream S\L Novel is I believe 'Desperation' for whatever reason it frequently comes up at what I would consider to be high GE prices. I am constantly tempted by this one, but know it's the start of a very expensive slippery slope (as soon as I get one, I will HAVE to own them all).
*shameless plug*
I have a copy of the multiple signed (all artists, publisher and of course SK) version of The Colorado Kid (it is a PC copy without number and also without traycase) currently on ebay. PM me if you are interested, we could work on a better price than it is listed for.