what i do when i buy in work
is work the round with the guy once, so that is half the money earned that he can keep (in your case 500 quid)
thats 500 quid off the total
i only buy work in for the amount its worth each time, thus 1000 is worth 1000
work it once and you only have to find 500 quid
see if he will go for 1500 quid, and let you work it once and he keep the money, then you only need to find 1000