Depends on where you are....the cost of getting someone to cut the glass in Podunk Arkansas will be a lot less than in NYC. I guess that's all relative
I promise you that it will take you longer to get it on and off the truck than it will for the glass shop to make the cuts and seam the edges. Having the box truck isn't the whole solution, though. You DO know that the glass has to be carried and transported on it's edge, like you were carrying a sheet of drywall? It must be laid at a nearly vertical angle, supported in many places and the bottom edge sitting on WOOD or carpet. NOT the metal flooring of a pickup or truck bed. Having two guys with vaccum cups (not the el-cheapo kind) will make it much easier and safer.