Heard knocking in AC compressor. Then stopped. No Cool air now. Cost to fix?

You will need to buy a compressor, accumulator (dryer) and orifice tube or expansion valve depending on your system. If you don't replace these components, most warranties will not be honored by the compressor manufacturer. They usually want to see a system flush kit on the same receipt as well. In addition to this you will need compressor oil, freon many auto parts stores offer a discount on these parts if you purchase all at the same time as a kit. And some will extend your warranty for by one or two years, be sure to ask this It's not something that they volunteer to tell you! I'm sorry I don't know the average labor rates for your area. But you should be able to go to the estimated cost section on repair pal.com to get an idea, just enter your zip code for shops in your area. If you are going to attempt this repair yourself, I would still take it to a shop with the proper equipment to vacuum the system down to -30hg. And recharge with proper amount of freon.