...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.
Video: Optoma HD3300, Panamorph UH480, Darbee Darblet, Samsung BD D6700 Blu Ray player, DIY Seymour Centerstage XD 138" Curved Screen
LCR - Tannoy DC12i, Sides - Tannoy IW63, Rears - Tannoy Saturn DC6ti Bookshelf, Ceilings - Tannoy CMS 603DC (TF), Tannoy CMS 601DC (TR), 3 Micro Marty/1 Full Marty DIY subs
Amplification: Emotiva XPA-3, Emotiva XPA-2, Outlaw Audio 7700, 2x Inuke 6000DSP
Receiver: Anthem MRX 720