It's hard to put a price tag on things when ebay is so unpredictable these days - too many 'shop' items in my opinion that aren't that much cheaper than retail thanks to the absurd ebay charges and silly new rules.
However, the arcade machine category in ebay uk is still fairly niche, so tends to attract a lot of collectors and restorers as good parts are harder to source in the UK. People never appreciate and account for the collective cost of the components in a MAME'd arcade, instead just seeing the several-hundred pound price tag and shooting for the cheapest possible (the fact you got it so cheap indicates interest would be low, regardless of spec - the PC never really affects the re-sale price).
I'd advise splitting it up to monitor, PC, interfaces etc. and put them all on low-start auctions; you'll probably get more than you paid if you advertise them right. Use a template similar to the one I gave to pboreham to sell his machine and mirror the points he made about the item for sale.