The facts don't support that. Expanding that logic, base model, "stripper" Camaros should be the best selling versions. They aren't.
People buying Mustangs and Camaros tend to be rewarding themselves, and tend to option out these cars more than most any other car category outside of the luxury and near luxury segment.
On top of this, traditionally Mustang and Camaros have had 50% and more of its buyers pick V8s. Both cars dabbled with 4 bangers in the 1980s, but couldn't pull it off.
Today, these 4 cylinder versions aren't being marketed as the "affordable" cars (seriously, if you cant afford a $30,000 car, youre probably doing your cross shopping new econoboxes and used cars, not new Camaros and Mustangs). Todays 4 cylinder sport coupes have turbos and easily tuned computérs attached to them and are marketed to generation raised on 4 cams, 4 valves, overboost, and computer games who now have real income to buy something with style, cred, and balls.
My sis bought a new Camaro V6 back in 2011. Had just about everything except leather (not a great option when you live in Arizona). She also bought it with a manual. She often raced (and beat) routine challengers at stop light Grand Prixs. My dad thought my Thunderbird SC was a race car, and my 02 B4C was insane. Current V6 Mustangs are quicker than stock Fox era 5.0s.
Now we have Ecoboost Mustangs and new Camaros with for cylinder engines that are at least as quick if not quicker than everything I mentioned save my LS1 (and the new V6 Camaro might even equal or beat that).
Point is, that although to those of us who keep up these things every day of our lives still have a hierarchy of performance relative to today, to the 99.9% of the rest of the population, the measure of performance is still measured by the seat of the pants. If something will smoke tire on demand, or it gibes them a peek of fear when they punch it on an onramp, or the car's still accelerating when they run out of nerve or space to keep the pedal floored, the car is plenty fast.
Has little to do with affordability.