well then it rather depends on what he is asking you to do.....and what else you are getting from the deal.
If he is expecting you to provide a certain level of service and make a concerted dent in the population from numerous hours spent out lamping/gassing/ferreting etc, and will blame you should his crops get damaged in the spring (which I've seen happen all too often), then I would suggest a sensible hourly rate, depending on how much you value your time.
(we used to employ a chap for around (£25/hour) but then he had full professional liability insurance, and relevant tickets etc and was a professional pest controller who charged extra for antisocial hours etc, ie, being out lamping or early doors......)
Personally I would rather he gave me permission to shoot the deer as well and cover any expense that way.