The Vega's reputation was sealed long before the Cosworth appeared, leaving few suckers willing to lay out nearly as much for a Vega as for a Trans-Am, Z/28, or even a low-end Corvette.
The Monza wasn't tainted as the Vega was...a Cosworth Monza would have been the ticket. Unique front/rear Endura end caps similar to the AMT '75 kit, or any of a few GM dream cars of the day, would have made the Monza look even slicker compared to putting gold pinstriping on a Vega...
--Mark