I replaced the fan motor on my unit, which incidentally looked something like yours, a few years ago myself. The fan motor died, apparently because of bad electrical connection (the wires were badly oxidized), so I took a picture of the motor plate with a digital camera (this was before they were included on everyone’s phone) and went to the closest electrical supply shop and showed them the picture. I got a new fan motor for something like $80 and installing it was easy with simple hand tools (just make sure you pull the breaker AND turn off the thermostat).
Using my simple calculation of prices are likely 20% higher than they were, your technician is likely to markup the motor about 50%, and labor cost equals part cost, I’d estimate it will cost you $300. Of course, labor rates will vary widely, depending on your location. I guess the take away is if you are comfortable doing it yourself, it’s pretty easy. If you aren’t, you had better call a technician.
Finally, you do know that it is going to wait until a Saturday afternoon on one of the hottest days to give up the ghost, don’t you? It’s probably worth a $80 service call to have a technician take a look at it while it is still working to allow them to schedule the repair when they aren’t super busy.
ETA: The usual life of a central air unit is about 20 years. You are due for a replacement. Usually, that means inside and outside unit replacements. If you own this property and plan on staying there for a while, you might consider looking into replacement. Yeah, $300 is a lot cheaper and it may be the best option, now. But, a new unit may be in your future.