In SQL Server what is the database cost for setting a default value on table that already has over 1mil records?
In order to apply this change the process will need to acquire a SCH-M lock and once it's got that it should be pretty instant (a metadata only change.)

The only risk is that if you have a long running transaction that is using the table whilst the ALTER TABLE statement waits to acquire the lock any other subsequent requests coming in that require a lock on the table will be queued up behind it to prevent your alter request from being starved.

So you could end up blocking production queries for a while until the lock is granted and the change made.