Radiator caps rarely fail but if you do need to replace the radiator cap then a new radiator cap is around $5.00 to $20.00 depending on the make and model of vehicle.
A new radiator cap is pretty cheap and auto parts stores should have them in stock.
Just go down to your local auto parts store and tell them your make and model of vehicle and tell them you need a radiator cap.
They should have a new radiator cap in stock and all you do is just remove the old radiator cap once the coolant has cooled down and then put the new radiator cap on.
If your radiator is bad and it needs replaced then the cost of a new radiator is around $100.00 to $400.00 depending on make and model of vehicle.
Hiring a shop to install a new radiator can cost around $500.00 to $1,000.00