If the issue only occurs once every two years, is it a better financial option to simply recharge it now yourself with a $20 kit from Advance or Walmart...and revisit again in 2014?
It's not going to get better. All slow leaks turn into fast leaks eventually. Since it is so slow he has time to plan things out but shouldn't just recharge every 2 years and pretends it's fine. What will eventually happen is it won't stay charged at all or the leak will cause some other sort of catastrophic failure. A failed hose or o-ring is already a given cost to repair and not that expensive. Why take the chance of adding a compressor, condenser, or something else to that list.
I would probably charge it for now, figure out what is wrong, and start planning to fix it when funds and/or time presents itself.
Most AC issues with leaking Freon end up being o'rings or lines. A quick look on rock auto gets me this for all motorcraft branded parts:
Compressor hose assy: $153
Liquid Line: $95
Drier/Accumulator: $57
My planned parts cost would be $305. You can also use a four seasons Drier/Accumulator for around $20 if you want. Plus if it's only one of the lines then that is more money saved. If you find it's just o-rings then you are out maybe $10 in parts for the offending o'rings plus an evac and charge. That's the best-case scenario. If you do find out it is just o'rings then buy the kit and replace them all. While you've got it apart might as well spend a few bucks to make sure none of the other ones are about to go. In theory you should replace the accumulator/drier any time you open the system but eh, that is a judgement call most of the time. I've heard of some people putting them in the oven at 250 to boil the moisture back out of them and then reinstall them back in the car. I always replaced them when I did major work since they act as a filter for the system.
Here is what I've seen "just refilling a slow leak" on an AC system also cost in addition to the above:
Compressor w/ Clutch: $390
Condenser: $190
Expansion Valve: $53
Again all motorcraft parts so cheaper alternatives are offered, but you get my point. A Freon leak is also pumping out oil that the compressor uses to lubricate itself. The compressor will eventually end up grinding itself to death while pumping everything between it and the drier/accumulator full of grindings. It also looks like these cars have a parallel flow condenser so you can't really flush them out well. Options are replacement for the most part. The expansion valve isn't really serviceable and you should replace it since it might have garbage in it preventing it from working right too.
