You can buy most entry level programmers for around 300 bucks. If you are patient you can get one used for 150-200. Most will show decent gains in mileage. It really all depends on what you want to do with the truck. There are some higher end tuners that will both increase mileage and power substantially.
I have been looking into an inexpensive tuner for better mileage, but ultimately decided to bite the bullet, hold off and wait to purchase an EFI live. As much as the other inexpensive tuners look attractive, EFI just has a good track record and I also plan to increase hp later on down the line and figure might as well start out with a great tuner.