How much does "BMW" coolant cost?

My guess is that BMW cares about quality and its good stuff.
Lets assume your paying $100 more it really does not mean much.

The one thing that is certain is that after 4 or 5 years new coolant is a good idea.

If your coolant level is down it may mean that you have a leak and it might be a good idea to check the hoses. It might be a good time to change the hoses.

Mixing coolant is not a good idea, yes its fine to get you home but its probably a good idea to put new coolant in the system.