Mostly flying schools charge by VDO, if one is fitted. What works out cheapest all depends on what you're doing. On a Xcountry with the power well up in something like an Archer, tacho can actually work out more than VDO. After a session of circuits or stalls, tacho is usually up to 30% less than VDO, which usually starts counting as a result of oil pressure and so counts from start-up. Worse, some start counting as soon as the master switch goes on.... Airswitches I've come accoss, either work on the airspeed indicator, which starts counting beyond a certain speed, maybe 40 Kts, or have a small vane mounted outside somewhere that blows a microswitch shut when the airflow is enough. So, any self respecting owner/flying school would be nuts to charge by airswitch - but would almost certainly use that for the maintenance time. Another variation is the one that starts counting when the gear goes up. I once knew a guy who had one of those. Was OK until he took it for a 100hrly where they jacked it and rasied the gear - then left it up over a weekend.
Charge by VDO, maintenance by airswitch - that's how they make money!
$215 VDO is less than some. 172's I know of vary from $160 - $245 VDO inc GST.