You are making a lot of assumptions in what you have said.
In recent years the hierarchy of The Showground has changed from the times when I used to deal with them a decade ago, so I can't speak about the current management team, but my assumption will be that the practicalities of running Speedway at the venue will not have changed very much, if at all.
Speedway pays significant rent to the venue. I don't know what the amount is these days, but when I was promoter it was, I felt, jaw dropping. It is considerable over a season. The Speedway club are also not allowed to bring their own concessions (catering) on to the venue, so the Speedway clubs income is restricted to admissions and the sale of programmes only. Speedway comes a long way back in the queue of the events held at the Showground and that's only rightly so, it is a business after all - actually it's not, it's a charity.
Effectively you pay rent for the key to open and close the place, you get little or no help from them in putting the event on or clearing up after. The Showground is not a charity (actually it is, but you probably get my drift). The Speedway staff volunteers do an absolutely amazing (and thankless) job in getting the place ready for Speedway and clearing it up afterwards, it takes a monumental amount of effort and I mean monumental and these people are the real heroes of the Speedway club. The Showground does not 'plug' the back end or keep Speedway 'afloat'. They provide a key. A very expensive one at that.
The one thing Speedway does do for the Showground is provide an event for LOCAL people. Practically every other event at the Showground is supported in the majority by people who live outside of Peterborough. Not everyone locally owns a Shire Horse, Truck or Modified Car or wants to pitch a tent a couple of miles from their home and there are only so many times you can drag yourself along to Truckfest. The 'dirty shale arena' gets plenty of use from other events, often these events damage the track and fence, which additionally costs the Speedway club to put right. If other events use the floodlights - which the Speedway club paid for and installed - and a light fuses, the Speedway club has to pay for this to be replaced as well.