I went to a local motorcycle shop that carried a wide variety of helmets, and found that they all fit very differently, favoring different head shapes. I found an Arai I liked and bought that. It was fine for HPDE. The next year I started racing, which meant getting a HANS-ready SA helmet. As it turns out, Arai is pretty consistent across models, so I simply picked out online an Arai helmet in the same size, and when it arrived it fit perfectly.
A proper helmet should fit snug like a ski boot. But not snug like an iron mask!
A racer-engineer friend explained to me another difference between motorcycle helmets and SA helmets: motorcycle helmets are engineered primarily for The Big Impact and then keeping you safe while you skid along the pavement, while SA helmets are engineered for repeated impacts that can happen when bouncing around inside the cabin (e.g., smacking the rollbar a few times).
As for price, I always liked the question: How much is your head worth?