They go for £30-£40 second hand on eBay, I got a brand new one for £50 on eBay.
Do you really need one?? Spend a day learning how to tune your drums! Save 30 quid and all the hassle that those drum dials are! :D
I've got one. Used it once. Always end up tuning by ear.
How much you looking to spend?
I think they're an incredibly useful piece if kit, exactly the same snare sound each time.