I just tonight put a bid in on about 350' of 18" wide trench (4" thick) as well as some other items. I got a quote of $1 per linear cut foot , so that would be $700 for the trench, plus $1.50 per cross cut. It will be inside with a slab saw. $6 a LF sounds crazy, but if thats what you can get all the power to ya!