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.