If you want a metal rail system (handguards), the aluminum ones from UTG are typically under $60 from CDNN, Cheaperthandirt, Sportsmansguide, etc... The UTG rail systems are well built and mount rock solid. The UTG, and many other metal rail systems, index on the barrel nut. If your barrel nut isn't installed with the gas tube at exactly 12 O'Clock, the rail system will be tilted to one side. I just recently encountered 4 Stag M4 clones where the gas tube ended up slightly past 12 O'Clock and the rail systems wouldn't mount straight.
Another option would be $30 Magpul MOE handguards (synthetic) where you can attach rails anywhere you need them. Magpul offers inexpensive synthetic rails and Midwest Industries metal rails that attach to the MOE. Along this line, you can just attach a rail to your existing handguards. UTG, Magpul & Midwest offer rails for standard handguards too. Another web site to check out is DSG Arms.
Just for example, here's a UTG handguard mounted on an M4A3
Magpul MOE on the same rifle... with a single MI rail mounted on the bottom front.