My cost was a bit over $100, but keep in mind it's HUGE - fits 4 Berklines - almost twice the area of some other plans I've seen. It's also overengineered. I'm sure I could've gotten away with studs every 18" instead of 12", and/or thinner plywood.
(11) 12' 2x6 - 4 uncut (long sides), 6 cut in half (6' studs), 1 cut into a 6' and 5' piece
(6) 10' 2x6 - each cut in half (5' studs)
(2) 1lb boxes of #8 3" wood screws
(12) 3"x5" sheet metal joints (dunno the right term -- see pics)
(1) small bag of #6 1" wood screws
(3) 4'x8' pieces of 3/4" plywood, each cut into a 4'x5' rectangle**, and one of the scrap pieces cut into thirds (three 4'x1' strips)
cordless drill with lots of charge!
1) Set out the first layer of sides & studs as shown in the pictures. Mark the side pieces every 1' using a tape measure and Sharpie.
2) Screw together with the #8 screws, 2 per stud per side. The studs probably won't be perfectly uniform; shouldn't matter given the 3" screws. (My first 1lb box was actually 2.5" screws, which were a little scary to use on short studs).
3) Here's where I deviated from the linked plans. Screw one of the remaining 12' boards to the top back side, using the sheet metal joints and #6 screws. I used 8 joints, 4 per side.
4) Now assemble the top layer of studs & the 4th side piece. Use the lower studs as guides. Ensure the two levels are flush before screwing; this structure gets its strength from studs resting end-on-end yet mostly.
[The linked plans say to assemble the entire top layer separately. I don't think I could do so accurately enough. Not to mention my expanded version is a seriously
5) Just for security, I used 4 more metal joints to join the outer studs to each other (top to bottom).
6) Lay out the plywood. The top pieces require at least 4 screws; the stair pieces should only take 2. Use your Sharpie marks and previous screw holes as a guide to ensure you hit the studs; don't try to remeasure! (made that mistake) Put your weight on each corner as you're screwing, so that no vertical movement or warping is possible regardless of weight distribution.
7) Paint or carpet.
**Should actually be 4' x 63.5". I calculated the inside width instead of the outside width, resulting in the 3.5" gap that's visible in the last picture. Oops. Oh well, I have plenty of leftover plywood to fill in the 12' x 3.5" area. I'm treating it as an opportunity to add a stripe to my paint job