It is a lot, but sounds about right.
6" lift is roughly $2,250, 37" tires are roughly $400 each so $1600 and 22" wheels would probably be $300 each at a minimum so at least $1200 for those. Comes out to about $5K just using rough numbers. A little more/less using actual product prices doesn't seem too far off from what to expect.
That is a lot. And if it were me, for that kind of money, I'd be finding someone with a similar configuration and checking out their truck in person. See how it feels to drive (or at least ride in), and get some real world numbers on fuel economy rather than internet speculation. We can all theorize here, but that's a big gamble, and not likely one you'd be able to take off and sell to recover your money if you don't like it. If it was me dropping that kind of coin on a mod, I'd want to be dang sure it was exactly what I wanted