Look at the handguards.
The cheaper one doesn't have a full pic rail on top, and has three screws where it clamps on.
The higher priced one has full pic rail, two screw at the clamp, a different pattern on the tube.
I couldn't tell you why they are priced $90 different though. Maybe it just depends what PSA had to spend on their materials to make them. Market prices for steel and aluminum going up and down?