It depends on the kind of idler pulley you are looking for and the car. Online it varies from about $40 to $60 for the pulley alone but if you are replacing the pulley, you may need to check the belt. The belt replacement will come in at roughly the same price.
Check garage prices
When you are replacing the pulley you may want to browse garage prices before employing one to replace the equipment. Unless you are doing the labor yourself, you will need to pay the labor time charge. This shouldn't cost too much but it takes just under an hour to get the job done.
There are plenty of websites that are very reliable for doing this job yourself and is isn't too hard once you get to know your car. The main concern with doing it yourself is ensuring you know how the belt is routed around the engine. If you don't, putting everything back in the right place may be tricky even if it is just the pulleys you are replacing.
Another cost cutting idea is to buy the pulley online and take it to a mechanic. This way you aren't paying shop prices or the delivery etc. If you do buy the parts out with the guides of a mechanic make sure you know exactly what you are looking for. Having a qualified mechanic will reduce the risks of damaging the car further and costing you more in the future.
New Idler part may be $30 bucks to $75 dollars. Procedure: Serpentine belt is removed, takes 2 minutes flat, and then your old idler pulley is removed / approximately 4 bolts. New one installed and fan belt reinstalled. Mechanic fee is $45 dollars an hour on average and the mechanic is going to take IN REAL TIME only 20 minutes to perform the entire procedure but may round it to an hours work, the way it is. So about $40 dollars for the mechanic and about $50 dollars for the part.