Its worth the money the first time you get in a black interior with no AC in 90+ degreees lol. I drove a fox with the AC removed for over a year and that summer sitting on the vinyl was BRUTAL. Also any traffic jam meant I was sweating balls and hating life. It was fast as a mother though...
What Vettn said is correct although, it is likely the hoses or a hard part. You'd see dye if it was an o-ring IMO. Often you can get an entire rebuilt AC assembly from ebay or wherever for pretty cheap and install is easy on a Mustang. I had the AC "black death" happen to my Honda CRV (2002-2006 were prone to it) and I was able to score an complete AC system for $500 shipped brand new. I'd imagine a Mustang would be similar or cheaper.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.