I usually visit a thrift shop once or twice a week. There are several of them in my area so I often manage to work a visit into errands. Most of them keep the "quality" bags in a case, chained down, or on rack behind the counter and you have to ask for assistance. These bags are generally priced higher.
I have seen nice bags, outrageous fakes, and genuine bags in terrible condition kept under "lock and key" and I often think that they are over-priced for a used bag. My rule is $20 or under for a project purse.
I don't want to spend much more in case they are fake or if I am not sucessful at restoring the bag. I usually stay true to my limit but I have spent as much as $45 on a rare bag in good shape that I felt pretty confident was genuine. I convinced myself to buy it with the "after all it's for charity!" argument!
Goodwill and Salvation Army will not accept returns - which is another reason not to spend much. The Savers Stores (called Value Village in some areas) will let you exchange an item if you bring it back with the receipt within 7 days.
For the higher-priced "lock and key" bags, I sometimes get lucky if they are having a sale or a coupon. A couple of weeks ago I stopped at the Salvation Army store and I saw a black leather
Coach Rambler Legacy in great condition on the rack behind the counter. I was attracted to it but it was $35 so I passed. I stopped back there the other day and "all items with green tags" were 50% off. It was still there and had a green tag so I got it for $17.50! That bag is still available from Coach for $260.
The bags I see most often at thrift stores are the older style leather Coach bags (but that's okay because I love them!) and the all-weather D&B bags. These more familar items are more likely to be marked with higher prices because they are more recognizable.
I have gotten some great bargains on less common items that were put out with the cheap plastic purses. I figure it is because no one realized that they were designer bags. All of my Dior items were put out with the cheap bags. I found that 46 cent Dior cosmetic pouch in a bin with a bunch of beat-up old wallets.
You have to inspect them carefully before purchase. You don't want to waste money or effort on a fake or on a bag that is just too far gone. Give them the "sniff test" and make sure that they don't smell like cigarette smoke or mildew.
If the bag has bad stains or the leather is damaged (piping worn through on the corners, etc.) it isn't worth trying to rehab. Pay attention to missing straps, etc. If it is missing a hangtag that may be easy to replace. If the stitching has separated (like the strap is pulling away from the bag) you can usually have it repaired at a cobbler fairly inexpensively.
Even if I am almost positive the bag is genuine I always take photos and ask for authentication here on TPF because I have been fooled by convincing fakes a few times.
You have to be willing to put time and effort into rehabbing vintage bags but that's what I like about it. I think it is fun to discover a lost treasure and bring it back to beauty and usefulness!
Just a word of warning, rehabbing is addictive! Good hunting!