I would imagine that the DOT rating isn't that tough to get. The Snell rating on the other hand, requires impact tests, flame retardancy tests (for SA helmets), etc. that are needed for actual "race" use.
Snell M (motorcycle) helmets can generally be gotten quite cheaply at most motorcycle shops (not as cheaply as DOT-only ones, but still cheaply). If you plan to go to Solo 1 or Club Racing in the future, though, keep in mind that Snell M won't cut it and you'll need a Snell SA (special application) rating instead (both Snell certs are legal for Solo 2 (i.e. autocross), though).
Shane -- SM 729