I understand after updating macOS, your Mac won't turn on at all, and just appears to be dead. Please review all the applicable steps provided below.
Begin with If your Mac doesn't turn on.
Read and follow each of its steps, including all the links within it, all the way through and including Step 4, which directs you to:
If your Mac starts up to a blank screen
Read and follow each of it steps also, all the way through and including the end, which takes you to:
"If you still need help, please contact Apple Support."
The estimate they provided is just that. They are unable to proceed without a hands-on inspection. The eventual repair cost may be more or less than the estimate.