That depends on what he is planning to put into your truck. A new Motorcraft "loaded" compressor will run you around $750 retail, after dealer markup, but includes the clutch, coil, pulley and is pre-oiled. Just the compressor should be somewhere around $660, though, I'm not sure about aftermarket stuff. If you want OEM, you might try calling somewhere like Bumper-to-Bumper instead of buying direct from Ford, as they sell Motorcraft without the markup (in fact, in Louisiana, when Ford "orders" parts that are not in stock, they typically come from a local Bumper-to-Bumper warehouse). Your mechanic may charge a markup if he buys it, so I would talk to him to see what your options are. Also, you may get a better warranty allowing him to buy the parts (some mechanics cover labor costs as well as replacement if they buy/install everything).
Keep in mind that the mechanic will have to vacuum out all of your freon before swapping the compressor, then recharge your system afterwards. He will likely charge for what he puts in as well, without giving you a credit for what he pulls out of it (I know that sucks, but it's the standard practice. They can't just reuse what's evacuated from a system.) Everything else will depend on labor and misc. charges. Hopefully someone with more recent experience can offer some additional guidance on price, but you may want to ask your mechanic what parts he intends to use. Good luck.