That sounds reasonable, but is it efficient?
1. It discourages "routine maintenance" -- it's a lot cheaper to go to the dentist annually than to get a root canal every ten, yet there are many who if they only had catastrophic dental insurance would skip the annual.
2. This "cost of living" varies drastically based on age, sex, pre-existing conditions, et cetera. So it couldn't be covered fairly by a single fixed UBI. So you'd need bureaucracy to determine who is eligible for extra coverage
3. Every small doctor's office would need a billing department. In socialized healthcare countries, "billing" is not much more work than filling in a time sheet, and is often done by the doctor themselves.
It's cheaper, more efficient and more fair to cover day-to-day healthcare for everybody than to just cover only catastrophic care.