Who the hell is selling a rotofab for $400+. It can be found for 300-350 all day long. FWIW, Most vendors here on C5 sell the K&N Typhoon for $250. Sounds like the GM March maddness sale is just a case of March moodiness.
FWIW, if you're looking for performance, there are MUCH better options than the K&N and for not a whole lot more $$.
The Halltech Yellowjacket is priced at $349 (maybe cheaper if you call the shop). For that you get 100% more average HP gain, 67% more peak HP, and 100% more peak tq.

Results per the Jannetty CAI test.