Usually, A/C shops would charge about $100 to test the system and charge it as well if there is no leak found. at the same time they will put in the dye for the leak to be found. Tha'ts for the indy shops. The dealer in NJ charges $199.
But I would recommend getting it done professionally for just another 50 or so dollars. This way the leak can be found and you wont have to buy the freon again incase it leaks away. The cost at the shop would cover refill of freon most of the times.

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