I don't know how the GT350 is going to be in the low $50's if a loaded GT is over $40k.. I did see a few dealers offering several grand off sticker though, but still that's a lot of cashesh for a GT considering how fast pony cars depreciate.
Actually, it is quite easy to option a 2015 GT convertible 10 50K.
2013 Mustang GT Premium Convertible
Candy Apple Red, Auto, FRPP Tune, GT500 Axlebacks, KONI Yellow Sport (adjustable dampers), FRPP Front and Rear swaybars, Custom Removable 4 Point Roll-Bar, BOSS 302 Low Expansion Brake Lines, Roush side splitters, GT500 spoiler, Redline Tuning hood struts,18" Ford Chromed Aluminum Wheels (9R3Z-1K007-A), Michelin Pilot Super Sports - 235X50X18
2011 Mustang V6 Premium Convertible - Gone