The green you're seeing is UV dye. The price on the manifold line is about right, list is like $214 or so with auxillary air (P/N XL1Z-19D850-BA. Along with a drain, sweep, and recharge, we charge 1.4 hours plus however much refrigerant you'd need. I did the manifold line on my F250 before, but then again I had the engine out at the same time
My rough guestimate based on what the stealership I work at charges:
Manifold line: $240
Drain, sweep, recharge: Approx. $105 labor plus $2/oz of 134a, plus taxes, shop fees, etc al.
Labor to install manifold line: I'd say maybe an hour, hour and half tops, using a relatively low $70/hr labor rate, $70-100.
I wouldn't recommend any type of sealer, during the summer when ambient temps are high, head pressure in the A/C will be relatively high as well and might make short work of most sealers. All things being equal, I'd call around to some independent shops and get a second quote.