I'd say about £5,000 for a quick sale - Cash waiting
Seriously though I think its worth a lot more than anyone would pay - a shame but thats the way it seems to be
It appears you have to give mods away unless its a TT or simlar then it adds half of the cost of the part (without the labour to fit it)
I'd be gutted to have to sell a car like that, but I think you would get more returning to stock and selling the mods.