Why do you think the cost of building the HD maps is prohibitive? How much do you think it would cost to say, keep a square mile of a dense urban HD map updated for a year? It's probably way lower than you think.
I didn’t say it was prohibitive but it’s certainly not cheap. The cost is per km rather than per square km. DeepMap charges $5k / km for example (the big companies don’t publish their costs, but it’s widely known in the industry to be on par with this). I can’t seem to find the stats again, but map invalidation happens on the order of weeks to months.