A 1x6 which is probably thicker is usually around $4-5 a 3/4 plywood at 4x8 is $51 ish, so 8*1x6 ($4.5 at 8ft length) = $36, and $51 for plywood. So no, plywood is not cheaper at apples to apples comparison. If you chop the thickness down you can get it to be comparable or cheaper. But plywood is stronger then a bunch of 1x6.

> A 1x6 which is probably thicker

A 1x6 is .75 x 5.5 inches.

Plywood is still more expensive with your numbers. OSB may cost a little less than the plywood though.

That would depend on the mill, you can get nominal and actual sized wood. At least I can in Canada get true 1 inch x 6 inch wood.

I think it has been for at least 30 years - perhaps longer. Structural beams were cheaper to make of solid wood when they were still lots of big trees to be had. That's no longer the case.