Replacing the car seat foam is usually cheaper than replacing the entire seat, unless the seat's cover is torn or the metal frame is broken. Even broken frames, however, can be welded back together.
To determine the best thing to do with a damaged seat, make sure the cover is not threadbare and then gently take it apart, stripping the seat down to the frame. Check for broken welds and springs. Springs and the upholstered covers can both be replaced, if needed, but remember to add the cost of replacement springs and new upholstery to the total cost of fixing the seat. When inspecting the foam, look for signs of crumbling, powdering or crushing.