My 05 Dak had a plastic drop in when I got (I think it was factory) and it was pretty cheap (one of the things that held it in broke off, although it never tried to come out on me like other guys are saying.)
For my 07, I bit the bullet and got LineX. I think it ran about $400 (I specifically asked for no over the rail since I wanted to make sure my tonneau would fit) and it has been really nice.
I have 2 (75 lbs) sand bags in the back of my truck (after the 05 got squirrely in the back and had me do a 270 spin into a guardrail on an on ramp, I like the extra weight back there) and every once in a while I will do something stupid and hear them go THUD up against the end near the cab, but no dents!
I've also had bags of cement, a big steel post hole auger, and a bunch of other stuff back there (hauled a bunch of bricks for my in-laws once) and have not noticed any dings or dents (although some of the spilled cement dust is still in the ridges - need to try a power washer on it someday.)

 

My drop in wore away the paint inside 6 months so I got a Spray-in and love it they even took the drop-in plastic one out for me and disposed of it for me. which was cool since I didnt want to deal with it. I got mine done by Millennium Linings in Indianapolis which around here is Line-x. They even put to reverse logos in mine but I didnt go for the color thing...

 

just pay a fat guy/girl to ride in your bed all the time....