In Formula A, ICA and ICC we would have racks of engines at European races, when racing for a manufacturer. But at the British Championships it was usually 3-5 and you knew ahead of time which two you were going to race. Sure some people turned up with racks but it didn’t make them win.
Additionally most of the time, they were off the shelf units that were tuned (within the regulations). You could do that yourself or you could pay varying amounts of money to a tuner to tune. And i want to clarify, I mean a tuner, not someone who just pulls out a standard piston, hones a barrel and puts a standard piston back in. One of my major bugs in the first years of rotax and x30 was calling the rebuilders tuners when they aren’t legally allowed (or talented enough) to tune.
Like Alan said, the initial issue with rotax was the variance in the tolerance of different parts meant you could mix and match parts to make a super engine. IF you could afford to buy 20+ engines and a dyno to run through all the combinations. Its supposed to be better now with tighter manufacturing tolerances but it might just be that fewer people race rotax now.
Tag onto, that you weren’t tied to a manufacturer or parts. There was nothing stopping you using IAME parts in a vortex engine (if it fit).
So in summary engines were cheaper, parts where cheaper, and you could do the work yourself if you had the skill set. I baulk at how expensive parts are for the commercial classes. You’ll always have people turning up with tonnes of material (chassis, engines, carbs) because if people can afford it they will. The difference with the old classes is you didn’t need to, you COULD tune your way better then the people who did have tonnes of material. I know of a few British champions who won on a tenth of the budget of the biggest spender and that was a super competitive championship.