'I'm in the market for a new HVAC system. I've noticed that the higher
SEER units jump dramatically in price going from 10 SEER to 12 to 14.
10-12 is an 85% increase. 12-14 is about %50 increase. '

The 85% increase is a little high if thats what youve been told. The
increase is because in 12 eer units, there is a Scroll Compressor over
10 eer units. It upps the price . They are quieter and usually more
reliable. Different design vs. reciprocating compressors.

'I've also been told that the 12+ SEERs have "variable" (valve can
open/close to change the pressure gradient) expansion valves. Lastly,
these "variable" expansion valves are prone to failure and frequent
servicing. Is this true'

Cooling Coils that have this TXV are more efficient and can tolerate
lower indoor load conditions than a fixed metering device can. TXV's
are generally reliable and typically last for many many years before
something goes wrong with them. I would not let this affect your
decision.