True height gauges are for making absolute measurements with true read-out of the item's dimension.
I sometimes find myself using a granite plate with a TDI and doing comparative measurements. EX: I have a 3" known gauge block, I'll set the TDI to that then, slide the workpiece into position and measure the differences. This in my opinion is a good way to tackle that issue w/o buying an expensive dedicated height gauge. You can also use the Z axis DRO on your mill to help with these kinds of problems. I've done it before and will do it again if needed (My main use for Z-DRO on a mill).
I'd love to own a good height gauge but on the rare occassions I need to make such measurements, it's usually because it must be dead-on -and I fear a cheap $50 gauge won't do the job. ... Now why one would cost $1500? I don't know... maybe it was very large with guaranteed high precision. For the HSM, I should think $150 - $300 with an 18" range is the sweet spot.
Ray