I set my limit at 10 miles, which is quite a distance when you are driving it! and a 20 mile round trip for a spray tan is far enough I think. Anything over 10 miles I will consider at extra cost, but wouldn't consider going much over 13/14 miles really even for extra money unless there were a few people who wanted to be sprayed at the same time.
The way I view it is this, while i'm driving a 30 mile round trip for £25 I could have sprayed another person locally for £20. I know you are not always chocka block with bookings but you really need to draw the line somewhere. Now if someone wanted me to come and spray them who lived miles away I would give them a price but they wouldn't want to pay it as there are normally people on their doorstep who would do it for the normal going rate. JMO.