Do yourself a favor and stay away from the DIY stuff. I work at a truck accessory shop and i see trucks with those DIY liners occasionally and they look like hammered shvit, nothing looks worse than one of those DIY liners after a couple years when it starts flaking off all over the place. The DIY stuff is just rolled on thin and has no tensile strength really to speak of.
LINE-X, RHINO, SPEEDLINER are alot better option.
Be warned though, if you want cheap then expect cheap results.
If you really are cost conscious then mabey a drop in plastic liner is what you need. Those can be had for 100-300 dollar range depending on the brand. We sell pendaliners here for 150. Spray in liners in this area usually go for 400-600 bucks.