Many things to consider here. The high cost of these lenses is due to the huge lumps of glass on the front, which would be largely common regardless if the image circle covered APS-C or FF. The possible saving from making the smaller elements in the back even smaller would not be significant for the loss in sales and need to have multiple lenses.
Also, the key point of EF-S mount is it allows a rear protrusion into the mirror area which the smaller APS-C mirror accepts but would interfere with FF ones. A long tele lens doesn't need to use that protrusion, so in theory you can create crop sensor lenses with EF style mount. I think most 3rd party crop sensor lenses would happily work on FF bodies. As weird as it may sound, I used the Tamron 18-270 on a 5D for a while when I had the wrong stuff with me on a trip out before.
Canon 1D, 300D IR, 450D full spectrum, 600D, 5D2, 7D, 7D2, EF 35/2, 85/1.8, 135/2+SF, 70-300L, 100-400L
EF-S 10-18, 15-85, MP-E 65, Zeiss 50/2 macro, Sigma 50A, 150 macro, 120-300/2.8 OS, Samyang 8mm fisheye