Is $4,000 to replace the HVAC coil a fair price?

I have a small leak in my Freon (410 coolant). I had it filled last year around this time. My system is 12 years old and is a Comfortmaker brand. We can't find the exact leak location. I was quoted $4k today for replacing the coil, but I'm looking into other options, including having friends do it cheaper or potentially getting a whole new system. I'm located in Kansas City and am looking for advice. It's a 7-pound system. I haven't been able to get my friends out to look at it yet. I agree that I shouldn't do something based on a guess; I'd like the leak found first. I'm wondering if it could possibly be the TXV restricting the flow of Freon.