Last year I had a 9' Anderson double sliding door put in. It required a 10' header, a 6' slider doesn't require that length of header first off. If the contractor is recommending that it requires this, better get another contractor. I had a crew of 3 guys, the entire job took 3 1/2 hours. Labor was in that $1500 range, so for one guy, that does sound a little high, but the construction trade in Calif has not slowed down for quite awhile and labor is staying up there. The header will cost him more that $100.