Macbook Air, early 2015, 11".

ElleninUS,

Hey! Thanks for the update.

As for the exact cost of service, that can only be determined by a technician after an in-person inspection. You should set up an appointment for service with your local store or contact Apple directly for help setting up an appointment.

This guide can give you an idea on repair costs: Mac Repair - Official Apple Support

Take care!

Hey ElleninUS,

Thank you for using the Apple Support Communities! We understand your husband's MacBook Air has a blank screen. We'd like to help.

Sounds like you tried some decent steps there. To clarify, can you hear any signs of the Mac having power when you go to turn it on, or does the keyboard light up at all? If not, then you're likely dealing with a power issue. See if anything here can help: If your Mac doesn't turn on and If your Mac battery won’t charge.

If the same issue remains, his Mac may need to be serviced. Take the Mac to your local Apple Store, Apple Authorized Service Provider, or contact AppleCare. See Find how to service or repair your Mac and Contact Apple for support and service

Best regards.