Depends on your grade, just how much soil disruption you would do. unless it needs a lot of movement, i just like to have an aeration done defore I hydroseed. Machine time is money. We have hills, loose soil moves. soil excavation requires a licensed contractor here.
Here spraying would have to be done by either a licensed individual for lawncare, or you would have to spray it yourself. I Don't know what you are having hydroseeded but I always sprayed with roundup 7 days before hydroseeding. by the time everything was dying the grass started coming in.
I really didn't answer your price question, but maybe i shed a little light on the way we do it here. Price really depends on your local contractors, local market, and time it takes to prepair the surface.
Grassman