8-10 times may be possible but rare, i'd say.
You're looking at minimum £15k.
£15k spent on advertising would generate an awful lot of work if spent wisely.
If you do decide to spend your hard earned £ with him, id ask for absolutely watertight contracts and perhaps a solicitors beady eye cast over the contract.
Are you desperate for the work? If not, play it cool. See if he's still got it in 3 months. Still play it cool. 6 months, still got it. Offer him 2 times.
I sold some work last year. I wanted 5x, i got 4x.
That seems common and fair.