It's been a long time but I think I paid $400 or so for it directly from Jacobs. It was one that they had used in some testing but still like new. I had to build my own bracket to mount it and the coil that came with it but it was still a pretty easy install. I think I paid about another $100 for the K&N filter kit.
I would love to find another 5+ mpg but don't know that it is possible without some major changes.
I have seen many devices like the throttle body spacer and don't think much of them. They started hitting the market way back in the mid 70's with the first oil crunch. I have doubts that any of them work very well if at all. What does it cost? I looked and saw no price listed for it. There is also the Vortex gadget that has been advertised on TV at times. I wouldn't think it would do much either.
What is your mileage with the 5.2L engine? Most I have heard of get 14 to 16 or so or not much different from my 5.9. My father in law has been buying Ford F150's with all the luxuries and has had a new one every year or two for his last 5 trucks. He never gets less than about 19 mpg with them. His 2004 has the 4.6L engine, is the full crew cab(4 door) and drives like a dream, has plenty of power and still gets 19 or better. When driving it, it almost feels like driving an 18 wheeler, it feels huge. I may even buy this one when he decides to trade again.