Translation services seem to cost around £30 an hour or maybe 7p a word, so you're talking about thousands of pounds for a 200 page (60000ish word?) document. I don't know if the person you mention is a professional translator - if so, no doubt it would be reasonable to pay him a bit less, but you'd have to trade off the saving against the accuracy/credibility of the translation. I don't know, but it seems to me that if someone challenged some conclusion you'd drawn on the basis of this document, you'd need to be able to defend the reliability of the translation.
Maybe you could ring round for the cheapest quote from a professional service, then either take them up on it or go back to this person you mention and suggest that you'll pay him, say, 30% less?