Detached garage about 20×24, not a stitch of insulation. All the walls are open no drywall, including ceiling. Estimated 1930’s built, asphalt roof, pitched roof. It was suggested the cheapest way in a prior thread was to lay an osb floor on top. Then use insulation batts in walls and ceiling, and drywall it. Therefore eliminating the need to insulate under roof. If I cover the ceiling there is enough room on upper level for storage. However I would need to build a floor on top. Wood is now $30 a sheet so I don’t know if this is still as cheap. So my question breaks down to cost in different scenarios. What is the difference in estimated cost, is spray foam worth it?

Scenario #1 Closed cell spray from entire garage. Cover walls, leave upper area open and store items on top without an upper floor. Basically what I do now. Would I have to cover spray foam under roof?

Scenario #2 Use insulation batts, build a floor on second level (osb sheets are $30 a sheet). Insulation batts and drywall everything. 

Spray foam is more expensive but how much more? I probably need 15 sheets of osb for up top right? that about $450