Anything more than five years is quite good going for such valves in my experience. If you keep your eye on the market you can sometimes pick up complete Honeywell valves at a very good price.
Some systems give motorised valves a fairly hard life. E.g., if you have a thermostat that uses TPI control the valves may be cycled on-off six times an hour, say 50 times a day for, maybe 100 days a year, i.e. 5000 cycles. So, five years use is 25,000 cycles. Weather compensated systems, if they are correctly set up, cycle less frequently hence less wear and tear.
Whether it's worth trying cheaper parts probably depends on how much the labour to swap them costs you.
Check the valve operates smoothly with the head off. If it's seizing up any new motor won't last long.