How do you figure out what planner cost constants to use in Postgres?
First, the costs are not supposed to directly relate to the execution time.  They're strictly relative; a plan which costs more should take longer to actually execute.  You can adjust sequential_page_cost in order to "tune" costs so that they're closer to milleseconds of execution, but IMHO that's a waste of time.

For 99% of users, there's only three cost constants they care about at all:

effective_cache_size: set this to 75% of available RAM on your system.  Done.

random_page_cost: if you're using high-end SAN/DAS, lower this to 2.0.  If you're using SSDs or on Amazon, lower it to 1.5.  Otherwise, leave it alone.

effective_io_concurrency: if you're on Linux and have a drive array, raise this to the number of drives (or mirrored pairs) the database is stored on, maximum 4.

Tinkering with the other cost constants is not recommended unless you have a full performance test setup to determine if the result is really beneficial or not.