The cost of the fuel lines should be minimal ($50) with about 25 ft. of line needed. Add in some new hose clamps and you are looking at less than $100 in parts for that. A carb rebuild kit can be had for about $50 . . . a set of new carbs are about $500. If the throttle cable is replaced, then the whole set of cables that includes the choke and oil injection cables will be replaced with it since they are all connected. I'm unsure, but I believe that set runs about $75.
The labor to changed the lines should be about 2 hours. The labor to change the lines requires the seat to be removed to get at the fuel tank, so that will add some time. The carbs will already be off for the rebuild so there shouldn't be extra time for that.
Rebuilding, removing, and installing the carbs will require about 4 hours of labor. The added labor time for the cable would be 1-2 hours.
In summary:
parts estimate $225
labor time: 7-8 hours x $80/hour (high estimate) = $560-640
total is about $850 on average. Yes, you are getting ripped off. Find out what the charges for the parts are, and compare them to the prices on the parts link at the top of the page. Then see how much labor time he is charging for for each aspect of the job. After you have that info, post another thread on here and see if one of the mechanics thinks it is fair.