Cost will vary depending on diameter and wall thickness. Whatever you do, don't pay full retail. Since you'll be buying several 20' sticks, lean on the steel wholesalers to give you their best price. If you don't like what you're hearing from them, you might also check with local fab and/or machine shops, since they probably already get better pricing than you ever will along with free delivery. Most probably won't stock DOM unless they do car stuff or hydraulics, but you never know...and just because they don't stock it doesn't mean they can't get it usually.