If you really take a look at the different types of motorized racing available to people dirt bike racing is just about as cheap as it gets.
I agree that it is still expensive, but it is nothing compared what the street bike racer guys pay.
$400 for tires, probably good for 1 weekend of racing.
Wipe out in turn 1? Thats 600-1000 bucks for new parts you just broke.
The AMA license is $50 and good for a year. I didn't buy a district membership, so I just buy a 1 day pass for $10 each time I race. Gate fees are $5 , entry fee is $35. Lets say I do 5 races this year. Thats $60 a race.
I easily spend $60 on the weekend when I at home sitting around, between beer, going to the bar, movies, etc.
Everyone has a different budget, so I understand where your coming from.