Must say that it seems to vary wildly on here, so you will only be getting my opinion & nowt else.
I have bought domestic & commercial work, the domestic seller & myself agreed on 1 & a half times the value. His work was worth £1000 every 8wks, so I bought it off him for £1500.
The commercial work I bought went at double its value. I think it was worth £260 or something, and I paid about £550 in the end. On another one it was worth £110 a month so we agreed on £220.
Going by the above if I was you, I would value my commercial work differently to my domestic work then price accordingly.
It's also very well establised, so this would help as well.
Minimum should be £6/7000 IMO..... maximum well......could be worth a lot more to some...so it could fetch £8/10,000.
Like I said there are some who will say you could get a lot more, but this is my experience of buying work.
Hope it helps