Exchange-Office 365 migration

Good Morning,

I just received an email this morning regarding the migration of MS Exchange to Office 365 with my scheduled migration date of 9/25/17.  As with most "informative" messages from Comcast the content is severely lacking.  If Comcast_Phil or one of the other reps monitoring the forums would answer the following questions, so I can make an informed decision, I would greatly appreciate it.

1.  As pointed out by another post, the email sent regarding the migration, states the cost would be "lower" than what you're paying now.  As email/Exchange was included in my original contract for service, what is the cost for migrating to Office 365 for email and or cost per mailbox?

2.  As my email is "run" from a sub-domain, also included in my original contract, how will this work with the migration should I choose to remain with Office 365?  Will that domain be migrated as well?

3. If I choose a different solution for email, how long will it take for Comcast to release and or forward the DNS for the other domain?

Thank You.