What is a ball park figure to replace a burned out motor for a fan on a central air unit on top of house?

If you can do it it will be far less money, the typical markup for HVAC contractors is 100% and the flat rate is based on $180 per hour.

No legitimate contractor can exist performing $300 service calls so it will be easily this if not much more should you have a DC variable speed fan it could be $1,400

get a make , model , serial # or the units motor info and get back to us.

It greatly depends on, the make, model, and btu rating. Larger units for larger area require larger stronger motors and as such they cost more. It would be very difficult to provide you with a reasonable ballpark estimate without more info but my guess would be 50-200 for the part.

I don't know how you diagnosed a burned out motor except to notice that no air comes out which could be because the capacitor is gone, bad relay coil or other causes.

500