If $0.70/W is the labor, not just to carry a panel up to the roof (or use a lift), but to measure, install brackets with screws into roof, seal penetrations, mount rails if used, mount panels... then it seems reasonable to me.
Installing panels is where all the work is.
You can purchase hardware for GT PV for about $1.00/W, maybe a bit more if RSD. Another $0.15/W for RSD box?
Installed system is likely to be $3.00/W
Hanging an inverter and putting conduit to breaker panel and through eaves of roof is minor. To me, most of the labor for that $2.00 or $1.85 over and above hardware is the panel install.
Obviously labor is more "per panel" than "per watt", but for one-off jobs, there needs to be a simple formula.
I might consider having a roofer install standoffs with flashing, lined up to hold rails. For my permit office's policy of no structural permit for < 40 lbs/attach point, about two standoffs per panel. So 16 panels needs 32 standoffs. Similar to a roofer putting in 32 vents or penetrations for a TV antenna. Get that priced, and the balance of the work is bolting on rails and bolting panels to them.
(I actually want the standoffs screwed into rafters not sheathing, and would do that myself. But I think through sheathing is considered acceptable.)
In California, purchase a worker's compensation policy if you have individuals work for you.
If you hire a contractor, verify his license on CSLB web site. Make sure it is for the type of work. See if he admits or denies having employees. Request for his worker's compensation insurance company to send you a document declaring coverage (which won't lapse without notice to you.)
Having worker's compensation insurance covering the workers, either your policy or contractor's policy, is how you avoid liability for injuries.
