I bought a generic 41% round up (glyphosate) 2.5 gal delivered for about $68. I use 1 qt per acre to kill grasses, and up to 2 qt per acre to kill broad leaf weeds better. I also use 1/2 pint per acre in my white clover fields to just knock back the grasses with-out killing the clover.

so cost per acre on average .... $7 ( $2 - $14)