>Is that an exorbitant amount of money for maintaining a (global?) digital map?

Absolutely - Google is just one player, Apple Maps, Bing Maps, OSM.

Charging consumers 300$/year for map updates would never work even if free offerings didn't exist - they are just targeting a small niche and optimizing (ie. even if they made the price 30$/year most people would still use phone maps so they might as well milk the market that wants to pay for their solution as high as they can). If there were no free competitors someone would drive the mass market price down way lower than 300$.

Considering standalone GPS units used to charge like 50 bucks (and now I think a lot of them charge nothing/bake it into the up front cost) yeah I'd say so. Garmin currently charges something like 100 bucks for their updates based on some quick searching.

The built in unit for your car is a captive market, they're definitely gouging. Especially nowadays when it's almost impossible to replace them with an aftermarket unit due to how integrated they have become.