It might be in the ballpark, or really high, depending on what route the power cable must take to get to the lamppost and to the desired switch location and what obstacles must be dealt with throughout (porch, sidewalk, driveway, etc). There's the possibility of needing to factor in drywall repair and painting to restore interior finishes.

Talk to the contractor about how he priced the labor and get a second bid for comparison.

High, low, or right on. Unless you provide specifics about the path the cable would take, the distance from the house, and any obstacles, there is no way to know. If you're handy, it might be very easy to do yourself. You've basically got to decide where the switch will be, get wiring from the nearest power source to the switch, and to the fixture by running the cable in a trench to the desired location (preferably in conduit), and probably pour a concrete base to support the lamp post. Is that worth $850 to you?

1. Get multiple bids.

2. Get references.