I've run across several options while looking for myself...
The absolute least expensive option would be a Bushmaster/Yankee Hill Rail Extension at $34.95 for the front ring, with a $14.95 Mini-Riser for the rear ring and rings of your choice.
Rock River Arms has a new "scout rail" for $65 that looks extremely solid, but it doesn't have provisions for BUIS.
Would you at all consider a free float tube with a top rail? This is the way I've been leaning because it doesn't change the height of your rails, and leaves you free to use any BUIS you want.
I'm waiting for KKF to finish his Gen2 modular FFT's, but Fulton Armory has a new "Phantom II" FFT that is available already. It's completely modular and you can add the rails as you need them, so you're not stuck with 4 full rails you don't need. That would run you around $80 for the tube, $20 for a top rail, and you could use that Rock River $40 mount to hold the scope.