45 dollars is for the flange. The header is 450, and you will need a 2.5" collector flange for your catalytic convertor... or a 2.5" cat.

I'm running this on my GS-R (it bolts right up) with a Type1 Racing 2.5" cat. The flange they're talking about is optional (I believe it is the one that bolts to the back of your block).