If it is r22 that would be about the right price I know the shops are charging close to $100 per pound, if the system was running I could see maybe 1/2 that in 5 minutes maybe on a hot day (that just sounds a bit fast to me) is your system a 5 ton? If smaller I don't think it could take 11 lbs of Freon unless totally flat and then it should have been pumped down. A very long line set may require more freon. As far as replacing the evaporator that would take hours to do. There will be a tag on your unit that stated the type of Freon and the required charge in pounds and ounces something like 13lb 8oz (I think that was the charge on a friends 5 ton I charged last weekend it took 8 lbs ) I do mostly industrial systems but help friends so I might be a bit more cautious than someone that charges several units a day. If it took 11 lbs of r22 I think the cost would be close to what the going price in Oregon is now.