lukemelo216 has the right idea about soil structure. A lot of that cut-out and remove is 'old school'. It changes the soil structure, and when the weed seeds are going to be covered by sod, they cannot germinate wihout light and air. Also, how are you dealing with the irrigation system -sprays? rotors? Dig anything sub-surface and it will destroy the system; must not be an issue. Another thing you could do to prevent weed seeds from growing after installation is to apply Ronstar-G before you lay the sod. And do spray the Roundup Pro on a sunny day. Here, I charge 300.00 minimum for labor plus .10 SF for front yard install and .15 per SF for a back yard. Add in material costs, 20% profit, any disposal, labor burden, etc. and presto.