It has to be cheaper since there are no bends.
To cut a plate is easier than to bend it perfectly, don't get me wrong for people that don't need a plate for doubles, and like the flat setup, it's good to have a discount for the features that they don't want or need.
Me, I don't use much my DR TPII (inherited it from my brother last xmas), I don't like the fact that no matter how low I strap the tank, I'll hit the 1st stage with my head every now and them, so the Jet Plate probably will have the same effect to me.
I like the "extra" clearance that the bend in my SS Plate and the STA gives to the 1st stage, that way, no matter how I tilt my head backwards, I'll never hit the 1st satage with my head.