As mentioned, it depends on the extent of the wear/amt of parts/work required. If you are going to do it, be a little weary of the cheapest places, you usually get what you pay for (though in some instances you can pay a lot and get ripped). If it requires the full monty - new guide inserts, new valve seats and valves, given the cost (you'd be looking at a good few hundred) it'd make sense to enquire about getting some porting work done at the same time. It'd raise the price, but if everything has to be replaced, you might as well improve it. It doesn't have to be full race porting, just work on the valve/seat angles and the bowl area will net big improvements.