SHendren:
I took a look at that eBay auction, and already the price has gone to a point, with the auction not ended, that you can pretty much find the filter from a retail source at a comparable price. Notice that the seller is charging $23. 00 shipping on top of the final bid. I'm planning to install the AFE setup very soon, and I got a price of $199 including UPS ground shipping from smokindiesel.com.
As to how much horsepower increase you will achieve, I agree with the previous poster that it depends on how much fuel is flowing coupled with the increase of air. Just the filter alone is not going to do the trick.
Having recently switched from the stock filter to a K&N that fits the stock box, as well as installing a new exhaust system, I can tell my truck is definitely peppier and sounds better as well. Since I haven't yet installed gauges or made any fuel modifications, I can't say how much better over what it was. Suffice it to say that the improvement was noticeable from the standpoint of driving the truck, which of course, is a pretty subjective standard.
Good luck with your truck, and whatever upgrades you do.