The back door on my garage is an 8'w X 7't opening, so two ~4' doors, I built the frame with 2x4's on the flat with a ~center cross beam and two diagonals each, all 2x4 and sheathed, outside with 1/2" ply and 3/8" G1S inside, and also I strung a couple of cables willy-nilly inside for chain, or other saw resistance. Insulated with styro-foam, Cost ? I built it in early 1996. Security [beyond the doors themselves: Dead bolts (barrel) 6" HD on top, and the hinge side, both doors. bottom ones are homemade from 1/2 CRS and 1/4"- 3" x 3" angle for the mount, both doors, 4" into the concrete.
Approx. Mat'l list: 9-8' 2X4's (2 doors 3-12' for 1) 1 sheet of 1/2" Ext. Gr. Sheathing ea. [and a strip for edge and or center over lap] 1 sheet of your choice for the interior (ea.) [You may wish for an additional strip for inside weather strip/overlap, some screws, and 6"x8" HD "tee" hinges [3ea. for my doors] and necessary locking hardware, 2, [Barrel] dead bolts with padlocks, ea. door and 1 gate type sliding dead bolt (ea. door) mine lock only from the inside so exit is tougher if they gain access from elsewhere and plan for this to be their escape route (less visible here). If the space is conditioned or you plan to, some weather stripping. And some decorative exterior cladding as wanted/required. I slopped silicone & paint into the screw heads of the hardware and sheathing [redundant as all sides are dead bolted and he lumber is glued and screwed from the inside as well]. Make it difficult, noisy, and time consuming all three things thieves don't like, and improve your odds of detouring/detecting/catching them! Harry