But "affordable" is very relative when you are talking about door to door racing. It's brilliant fun but like most things in life that makes it very expensive.
Club racing (non CAMS) will probably cost $5-20k for the car + $500-1000 per round in tyres, fuel, travel etc.
CAMS state racing $20-50k for a good car (or 200k+ in sports sedans) +1000-5000 per round (depends how many sets of brand new tyres you can afford per race). I run at this level
National race series $50-$200k to buy a car, and say 20-50k a year to run considering travel, crew, tyres/brakes/oil/fuel, maintenance and repairs.
Supercars, Porsche cup or Superlap $200-500k for the car and 100k+ per year to run.
Tarmac rallies 50-500k for a competitive car + 20-100k for a full year (3-4 rallies).
So...I guess the main thing is to be realistic about where you can fit in. If it is under the club racing budget stick to track days and enjoy them....if you can't afford track days enjoy motorkhanas at $50 a pop.