Honestly... not a huge amount. Yes there are a lot of accounts but revenue on them is very low - especially the past six months - and the bottom line is what people often look at. Unless there's one or two big accounts skewing your numbers that we could look at, but Bet365 don't give you that information.
Personally I wouldn't buy the account myself - too much risk. As it stands, worth maybe €2k (based on my low estimate of value given previously minus the negative balance).
If you can wait it out a few months, start to see some better revenue numbers and clear that negative balance off then you'd have a better shot at getting more for it. But as it is it's super risky for the buyer.