Unfortunately, it really isn't that profitable.
The meeting that you are looking at for £15 admission is a 'domestic' meeting which will probably get a crowd of 500-100, so £7,500-15,000 admissions.
Drivers will total 20-30 per class so about 90-100 for a 3 or 4 formula meeting, usually around £25 per car to enter, so about £2,500.
A long way off your £40-50k estimate, and then you need to deduct the costs of putting on the meeting.
The 2-3,000 crowds that you have heard about are for the big championship meetings or maybe 1-2,000 when F1s are on. The difference with F1s is that they don't pay to race, they actually receive start money and prize money, and also get 3 free admissions per driver for their mechanics, so the crowd figures will include up to 200 non-payers. The payout to drivers for an ordinary 40-50 car meeting is in the region of £6-7,000.
I'm not saying there isn't profit to be made, but it is nowhere near the figures you are suggesting.
As an example, most of the meetings at Buxton (High Edge) are for the 'domestic' formulas rather than the national ones. The promoter has openly stated that he has to run long meetings with 6 or 7 formulas just to break even, as the drivers paying to race plus their pit crews and families paying admission will just cover his costs of putting the meeting on. Any paying spectators (often less than 200) is profit, plus profit on food and drink, but many people take their own rather than buying it.