My AC unit is 2 years old. I turned it on for the first time this season and it kept cycling on and off every 30-45 seconds. Never putting out cool air. The repair service replaced the refrigerant, 5 lbs, and it works just fine. The technician states I have a noticeable leak in the attic and need to replace the coil. My cost after warranty picks up first $500 is $685. Does this sound legitimate?

Usually the coil is warrenteed for five years so it should be free.
I do not understand the my cost after the first 500.00 is 685 deal?
Something is wrong with this picture and I would contact the manufactures office or local rep and talk to them about it. NO coil that is properly installed should leak after 2 years.

You may have just purchased a parts warranty on the system, that would not cover refrigerant and labor cost.

The warranty I have is $500 toward major parts, no reimbursement for labor. Is it common for the coil to leak on a 2 year old unit? And what is the average cost of replacing coil?

Is it common for the coil to leak on a 2 year old unit

NO

No its not common but it happens that's the reason you buy the extended warranty to protect your investment. It only takes one service call to make up the difference of the parts and warranty cost.