I don't know about prices but you find them advertising on saddlers notice boards /local horse press . Or ask at your yard if anyone there could do it for you if you paid them. If you re going to be clipping regularly or more than one horse it's probably worth investing in your own clippers. It's very easy to do it yourself if you have an amenable animal.
I had someone clip my horse last year for £25 - first time was a blanket, second was a hunter. He was fantastic. I found him through a postcard in Countrywide, he's a professional clipper and does racehorses mostly and was quick, patient and efficient.