AirTags would probably be the cheapest option for relatively reliable motorcycle tracking. However, there are already devices that can be connected to the bikes battery, hence basically run for ages, that also connect to GPS and GSM.

Tracking with AirTags assumes the thief (or someone closeby) uses an iPhone AND has the 'Fine my' feature activated, which is, given how motorcycle thieves operate in some areas of the world, is relatively unlikely. So you're probably still better off using a GPS+GSM tracker