When the LAA 737 and 321 had 40 inch pitch, personal TVs and decent under seat storage, I would pay for domestic F on many longer routes (MIA-LAS, FLL-DFW-LAS, etc) when the premium was reasonable. Now that more and more narrow bodies have been “Oasised”, I move that spend to Delta where available. Now I only purchase F domestically on the 757 or widebodies.
My mental calculus is I will pay $40-50/hr for F over economy for flights after noon (rarely pay for F on breakfast flights). So if MIA-LAS is $200 in economy, I’m willing to go to $450 or so for F. The lowest F fare is often in this range. Sometimes cheaper from FLL depending on time/season/routing.