I know some people do seem to have this view that guided stalking should be "affordable" and in my view it is - If you do even some simple sums you'll see how quickly the costs mount up on an average month.
If you've got a few leases you might be paying £15k a year (making it up) for all of those - Thats £1250 a month just so you have somewhere to run your business from to start with! Sure, some people might pay more and some might pay less but its pretty unlikely you're going to be paying nothing for all of your ground.
Add on vehicle lease, insurance and fuel and you're probably hitting another £4-500 a month, commercial insurance at £50 a month, accountant is about £1200 a year so call that a ton per month, 15-20 rounds of ammo plus any other adhoc maintenance of seats, larder, facilities, replacing/repairing clothing etc and, straight lined, that probably averages out at £2-300 a month...
That little lot adds up to £2080 in unavoidable monthly costs just for you (thats basically £25k a year by the way..), I have no doubt I'm missing a whole load of other things off and then theres the other things mentioned like additional guides you might ask along to help you and so on.
And dont forget you're actually trying to make a living and pay mortgage, bills and save for retirement with this business as well. Lets be honest any of us running a business like that would want to be billing at least £70k per year with a cost profile like that, especially living in the south with higher house prices. And then if you have kids to look after as well...
Work that out on the basis of 260 working days per year, less 20 for holidays means you need to be making around £300 a day the other 240 days. Now that sounds easy but you might not get bookings on every working day (stalking on a Tuesday or Wednesday anyone?), or you might only get 1 person in who doesnt get a shot and you only take home £170, so now you need to make an extra £130 another day to make up for it.
In light of all of that I think £85 an outing plus a kill fee is pretty reasonable TBH, and gamedealer rates on the venison is more than fair. Trophy fees are fair as well if you have said to your guide you're looking for something a bit special, and as Malc said if you have made sure you know the fees up front then you're not going to get a surprise after you pull the trigger.
As others have said - Find out the costs up front and make a decision on whether you yourself are happy to pay them.