What is the charge for diagnostics for an out of warranty Apple Macbook Air in India

Only the Apple Authorized Service Centre or Apple Genius Bar can provide the Accurate Costs.

It would be conjecture from our Remote Locations on the Globe to even guess.

After all, we are only follow users helping other users and do not have Direct and Physical Access to the problematic computer

Diagnostics by Apple or an Apple Authorized service provider are normally free. You are free to then accept or decline the repair once you get the estimated cost.

Have you seen this article? Mac Repair - Official Apple Support (IN)

It looks like it depends on the provider / service required: "Contact your local provider to inquire about available service options."