It vary's wildly depending on vets and what equipment they use.
I think my vet is in the region of £100 for just turning up with his snazzy digital machine, and then about £15 a picture from memory (this excludes call out fee and any sedation you might need)
When you say leg - it depends if you mean whole leg or jast part of leg! depending on the issue you should really allow 3-4 pictures per joint to cover all angles, 2 will normally be fine for say mid-cannon
mobile x-ray for both fronts, cost me about £150 including call out charge and sedation. That was for 4 different slides, one side view and one front view of each foot.