It really depends on the quality of the work.
If its taking one man only one day to earn 100 pounds, then not a lot, if its one man one day to earn a thousand pounds, then it's worth a hell of a lot more.
If there is only one customer, nothing at all as you could end up losing it. But something with a decent number is OK
I always think is it a lada or a Bentley, they are both cars that will get you from A - B, only one cost a lot more.........
Average I would say for 50,000 pounds per year would be about 25,000 pounds. Like I said though it depends on the quality etc