If you can find someone who does it on the side, meaning NOT a proshop, maybe $12.00 - $20.00. There is a guy on my league who strings racquets for $15.00 for others as long as they just use the regular gamma string he has. I am sure his price goes up if he has to use gut or something.
I string for people now and then. I usually just charge $10.00, but I usually only do it for friends, and they usually bring the tennis balls more often when we hit.
Pro shops can be pricy! In college I spent $25.00 a pop for cheap crap 3 times, then I got my own stringer.