People will probs disagree with me or say you can get more but if you want a real fair price, NOT the overpriced JDM part crap (just look at the 2nd hand ARC in ebay, £550?! You can get them new for that!) aaanyway a fair price, IMO would be about £350. Purely because it would really need tweaking when fitted to another car and a map often costs about £300. Then the fact you can get K100 supplied, fitted and mapped for under £700.
Edit: I have nothing against people selling rare parts for a bit more or if they need the money. But some stuff is ridiculous.