It's a tough call. When I look for rehabs, I look on CL and find the birds that are the most neglected. The more neglected they are, the more my heartstrings are pulled.
Every once in a while, there's a bird that just needs saving. At that point, I've emotionally and financially committed to the bird in my heart. I can't bear to leave them in the environment.
The CL fees are all over the place, but at least in Florida, here's what I've experienced [not all of the rehabs I've done, but the most memorable ones]:
Macaw: $650 asking / $550 paid (Pretty Birdy)
Macaw: $500 asking / $300 paid (Duke)
Grey: $600 asking / $400 paid (Rescue / Rehab / Nasty biter)
Cockatoo: $900 asking / 700 paid (Plucked clean)
Conure: $400 asking / $350 paid (Biter)
It all depends on what you're looking for. I find the best way to deal with a CL ad is to gauge the seller based on their level of desperation to get rid of birdy, how much they've neglected birdy, and their financial situation. (It's not hard to figure out).
For me, it's about the bird. I've told people a couple of times I would call the police due to the way they've treated the bird. Once you have a picture on your cell phone, the evidence is in hand. Those people will usually give up the bird for fear of getting arrested. Others will try to negotiate, and some are just looking to feed their drug habit.
I've never paid asking price EVER.
It really depends on what the deal is with the bird, but if birdy is driving the owner nuts and they're one that just got in over their head, they're easier to deal with.
I've also found that most macaws that are screamers when I visit them tend to be the most quiet ones when I get them home. Maybe they just realized they were saved.
For me, it's easy to work the "bite" and "aggression" out of a bird, but then again, I'm just used to being bitten and know when to give a bird time and space..
Don't get turned off by an aggressive bird or even a loud one. Definitely don't get turned off by a plucked bird or one that is obviously very abused. Those are the ones that need saving the most.
I have no love for CL people. I think the majority of them are the dregs of society. You just have to understand the mentality and if you do, you'll always figure a way to get the bird for less than what they're asking.
Good Luck!
Also, if you're open to transporting / picking up birdy, Florida seems to be full of neglected macaws.