I checked out both DT and ER. . . for a set of 225/45/17, ER seems cheaper overall right now. At DT, it's $120/tire ($480), plus $12 for valve stems, a $4 "environmental fee," and $38 in NV state sales tax. Ground shipping is free.
$534 total.
Edge Racing, on the other hand, is charging $112/tire ($44

, and ground shipping - at least to Dayton - is quoted at only $71. Apparently valve stems are included.
$519 total.
I am curious about Discount Tire's "Certificates for Free Replacement," though - with the way tires get worn while autocrossing, is that even worth it?