The price moves around quite a bit on OSB. I have seen it as low as $13/sheet, but it's usually close to $20. I did my roof in 1/2" ply because it was cheaper than 1/2" OSB buy a dollar or two. Last year I bought about 20 sheets of 23/32 ply for $20/sheet. Smoking deal. I built the sides of my big wood hauler (C70 10 wheeler) with it. Was going to do the same on the little hauler, but it's up to $34/sheet.
Yes, the ends are 2x4. If you use 2x4 for the span they won't sag. If you align the weight perpendicular to the grain. I have had 1/3 of a cord seasoning on with this method for just under 2 years. There has been no sag with only using 2 cinder blocks.