<speculation alert> The HSI G5 will probably require installation of a magnetometer at the very least, which could be troublesome to get passed the compass swing. My guess is these will end up near wing tips like so many are, so adding costs to running those wires. I'm guessing also that this will have a standby battery just like the AI G5 has, and will require the capacity test be accomplished per the Instructions for Continued Airworthiness also like the AI G5. The capacity test is dirt simple but each battery is $150, so if you had two G5 and both batteries failed at an annual that adds $300 to the bill in parts.
Everyone who buys should be aware of these details.
(see section 4 Airworthiness Limitations)
http://static.garmin.com/pumac/190-01112-11_01.pdf
I'm not sure what the standby battery for the Aspen costs or how many years they last.
The limited G5 documentation already mentions the magnetometer and location limitations.