Is there a cost to upgrade v15 (licensed) to Frammaker 2020?
If you already have a subscription you can switch to the current version without any additional costs.
However, if your version 15 is your own bought license, then you have to subscribe.
As I recall, when Adobe added subscriptions for FM, the perpetual⇒perpetual upgrade path was dropped.
If your FM15 licence is perpetual, you might as well keep it around. It can co-exist on the same machine as FM2020.