I was speaking to one top Championship level rider that was doubling up about 9 or 10 years ago and I remember him saying in a busy season the previous year he had earned just under £120k. But, when his machinery, tuning, mechanic costs etc were all taken into account they had cost him £109k.
So, basically he had risked his life racing speedway all over the country for around 7 months to be left with £11k. When you see it like that nothing makes sense, which is what I said to him. With a wry smile his reply was; "Yeah, you are right, but at this stage in my life it's better than having a real job!"
I think that sums up how costly it is and why they do it. Although, I bet the amount of money that ends up with engine tuners over a year is quite eye watering!