1000-1500kg,
15 quid.
That's if you want
repeat visits - either for the proverbial £100 burger, or for meeting up with people.
People travelling on business would pay more (say £50 if there was an ILS - that makes a huge difference to business flying ability).
If the airport was in some fantastic location, where the typical visitor visited just once every few years, then you can charge more. For example Prague LKPR was getting plenty of (upmarket) light GA when they were around the £60 mark, in 2005. Now they are more like £150 and hardly anybody goes there, leaving the ~ 10 staff in the "old terminal" twiddling their thumbs. I fly there, and to other £150 locations, every few years. I cannot think of any suitable "desirable" examples in the UK though.
That said, there is a big section of UK GA which won't pay more than a fiver, but you can't please everybody. These are mostly homebuilt/microlight pilots, and they fly a lot between strips. It is an interesting and valid business case whether you want to attract sub-1000kg traffic at £5 a pop. GA presents very little in terms of variable costs so all of that £5 will be a contribution towards your fixed costs - all the time you are operating below max capacity
And if you set up a separate "GA entrance" for them, they are not going to push your capacity, so that money is all profit.
BTW, there is (I understand) no known case, since WW2, of an airport fire service having saved a single life, in the GA context.
That would require a big change at government level, comparable to the govt giving a grant to the Royal Yacht Club on the basis that the yacht owners pay a lot of income tax (which I am sure some of them do)