Don't mind you asking at all. The cast carb is $375 plus $24.99 shipping for a total of $399.99. What I meant by "took a step down from a 40mm" is that APT's Buying Guide suggests a 40mm carb for a 500cc motor.

There a lot of variables that influence the proper size carb for a given engine displacement.  The amount of power an engine makes is not always the deciding factor.

A stock YZ 125 has a 38 mm carb and makes 28 to 30 RWHP.  A stock LT 500 has a 38 mm carb and makes 45 to 50 RWHP.