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The TrickFlow and Professional Products upper plenums are the same, I have seen them "live" side by side at a swap meet in Daytona. The Pro Products units are all over eBay...
Good info on that table cliffyk posted. I guess the TB is definitely worth getting, it's just not worth getting and keeping the stock plenum. More of a CFM gain from adding the TB than there is from adding the plenum...if you add it after the plenum upgrade.
Summitracing has them for $99.95, or $250 if you want to throw in the TB with it.
The trick flow combo on Summit is $250. Prob the best buy & Def worth it; you may choose either the T/b 70MM or 75MM along with the Plenum. I do agree that I like the TF plenum but I went with the BBK t/b. I believe it has a little better response that TF has with their t/b.