I thought the idea was to encourage the youngsters to come and try it, most with a parent. So if the kid likes it his parent might take him more often.
A quick look at the Zoo prices... Little old Banham Zoo in Norfolk is £57 and Colchester is £80, both for a family of four. Add to that once they have been once it's not somewhere you would want to go every week or 2..
I know speedway is expensive, but your not looking at the same Baboons arse every week at speedway.