Agree with others that it’s totally situational and can vary wildly. Generally I say that if it’s an acceptable price to you, book F outright. That way you have a true F ticket and more legs to
stand on in that respect with regards to IROPs, schedule changes, etc. (Not that it should really matter, but Delta IT is not that good in these areas).

That said, I did recently book Main and upgraded to F afterwards via FCM for a reasonable amount. Booking F outright was not an option as it was an Amex Plat Companion Cert fare. While I did not check the FCM price before being auto-upgraded to C+ as a DM, I did notice that after I upgraded to F via miles the receipt emailed specifically says “CMFT+ FRST upsell” so I’m curious if the rate would’ve been more if it was a “MAIN FRST upsell.”