Can't really recommend a low price tonneau cover, stick with the name brands, i found when i purchased mine a few years back that there were dozens of companies and the price was about the same for my model, approximately $300. Sure wish i could remember the name. Whatever you do don't get the kind that stick with velcro, the price looks great but they never seem to last very long before the stick-em on the velcro that attaches to the truck comes loose. I would recommend the snap on variety, generally the rails bolt right to the truck without the need to glue anything or drill anything.
As far as gas mileage, i saw a improvement of about a mile gallon or two. I have a 94 chevy k1500, 350 engine. was getting about 17-18 on open highway, now it's up to 19-20