...or you could build your own using Seymour AV's Centerstage XD Material and Aussiebob's curvature spreadsheet. I built mine (same size as you're looking to make) for less than $500. That's without masking, though, unfortunately. You did ask for a good bang for your buck option.
FWIW, I have seen others rig up a DIY motorized masking system as well.
Yeah motorized masking is where prices get really crazy. I thought 7k for a Stewart Microperf screen was bad... But motorized masking prices are just insane. I obviously don't get what makes them cost so much.