The other thing to consider is that those 450 4$'s are big money/big power engines. No one is out there is on a box stock motor. Even with all the hype about Chad Reed being a "privateer", he invested "millions of dollars of my own money" (in his words) in setting up his team and had a lot of help along the way. He especially noted "engines from Pro Circuit".
Not that the 2T's are not just as expensive. I remember when Ronnie Tichenor went to race in Japan. He said that once he got off the factory ride, it was IMPOSSIBLE to get those power plants and it was impossible to compete without them. He went to Japan because there he could make a living at racing. Even with the AMA's claiming and production rules, you couldn't get the tech. Eventually, Pro Circuit and others started building equal or near equal equipment but it is still out of the price range of most Pro riders.
Motocross may still be one of the few motorsports that the driver/rider is the largest percentage of the equation but at the pinnacle of the sport, the percentage that is equipment is greater. There are 4 parts to the equation but it must all equal 100% to win; Talent, Ability (strength, endurance etc.), equipment, and luck. Drop 5% and you are back to racing Arenacross.