Id expect to pay somewhere in the neighborhood of $300 or so for a garage to change it.
You are really paying for labor, as the belt itself is fairly cheap.
Recently had the timing chain gasket cover replaced on my 1996 Taurus at a very reputable garage. (Been using them for about 2 years with ZERO complaints...and Im picky) It ran be around $375 to have the gasket replaced and the timing chain. Mechanic said the chain looked it good condition, but I opted to change it since he was already in there.
But, by all means, when in doubt have it changed. I personally haven't had a chain or belt go out, but I've heard the horror stories. Its definately something I do not want to encounter.
Service manual on my wifes 98 Camry says to replace the BELT every 90K.
Since you bought a high mileage, belt driven, car with no service history I would have it changed as soon as I could afford it and consider it money well spent.