I'm going to have to agree with Craig.
Class 1 Autograss racing.
It does get any cheaper than this, in fact it's so cheap it's almost free!
It's a nationally regulated sport, (NASA) the British Nationals are held in Wales every September (I think) but rumour has it that this year will be at ROCKINGHAM!!
So, what's it take?
Purchase No. 1 - get a reasonably sound MOT failure Mini.
Purchase 2 & 3 (The most expensive items) will be Helmet and belts, Oh, and a race seat if you're at all worried about your back!
(Any aftermarket road-spec 'competition' seat will do: Cobra SFY, Corbeau highback, etc.) The only other purchase, (and some make their own) is a rudimentary cage.
(And lets face it - you're gonna have to buy a lid, set of belts, cage and trailer for whatever
The rest is easy: there is nothing else to buy!! Strip it 'til it's regulation friendly (No glass, interior trim, chrome, etc)
No modifications AT ALL are permitted, (keeps a lid on prices) and the grids are FULL, so they must be doing something right!
Using second-hand safety items, there are guys out there that are racing every other Sunday for an entry fee of £3.00
(guaranteed four races) in cars that can be as cheap as £250.00 all in!
Now that is c h e a p!
If anyone is interested in regs, I can point you in the right direction.