DB
I think the major difference is in the fact that the Waddo show is in effect in-house. Therefore the overhead costs are bound to be cheaper.
Also - don't forget that RIAT has no home as such. I am sure that the insurance costs for lodging at a USAF base as opposed to your own base are far greater. (not least becaude of the war and the security consequences).
As I keep saying - RIAT is the showpiece and the launch platform for other RAFBFE fundraising. RIAT has NEVER been a huge earner for the fund.
Either way - you have to think of it as a whole: At least they are putting SOME money in - and all the crews get to have a great time, do they not?
We could ask the public to simply drive past the gates and put a tenner in a bucket - now that would raise some money but be a whole lot less fun.