It depends on what you're having done. If they've got a mobile x ray machine, a call out for that can be cheaper than taking them in & using the big one. Best bet is to ring the vets & ask what they charge per film. My mare went in to have both hinds x-rayed from above the hock, from all angles a few years ago & it was in the region of £500. Friend had one or two to confirm a hairline jaw fracture & iirc it was around the £100 mark iirc. Did have call out & some drugs too & I'm sure total bill about £150 a few years ago.
I have just been quoted £300 for x-rays of the spine from my vet. All 4 feet were £350 when I had them done recently. All done at home with a mobile machine.