How much for a Gear Vendors?

[custom10nut]

I’m toying with the idea of putting a Gear Vendors Overdrive in my truck.
As a little improvement in Gas mileage (Especially now) would be a plus, my main goal would be the ability to cruise at 70mph on the Highway, without turning 3500rpm.
It’ll save on the wear and tear of the engine/trans (both rebuilt originals)
Has anyone put one in their truck?
Where did you find it? I
Is it difficult to install?
What’s a round figure of the cost?
Does the benefit justify the cost?
I’ve considered a 700r or 4L60, but don’t want all the electronics to control it’s function.
All responses will be helpful