Totally dependent on my finances and what I have in my pocket, including what I had to trade with and cash. I'm not an impulse buyer, I've learned one thing in watchin' my mom and her bf horse trade for 40 years. Everything comes around again and if someone bought one, there's always someone to sell one. You may just have to wait.
That said , I went to the ECCKS back in the late 80s and had no idea how much I'd spend and because it was my first big show everything was an impulse buy. I dropped $600+ that day and walked out with 2 customs and a funny little knife with a clip and a hole in a hump on the back of the blade. I went to get a new kinda ceramic v sharpener and he was sellin' this odd lookin' knife.
His name was Al Kessler, no it was Al Glesser, no Sal Glesser, I'm sure of it. I wonder what ever happened to his ugly little knife and his company named after a bug?
I've dropped $30 the most at a flea market, generally a $5-$10 impulse buy is a lot for me at a fleamarket, yard sales are even less. That's the nice thing when buying older traditionals, you can find beautiful knives in that price range if you're thorough and know your old knives, patience helps too.
A gun show I go in knowing what I want to spend and I have a routine for shows, I'll walk the show floor once scanning looking for piles of older knives and if I see something I'll keep an eye on it to see if it's garnering interest. If there are people around it lookin' at it I may have to hasten my routine and make an offer or show an interest and get a better look. Then I take a more thorough stroll around and talk to some of the guys I know and make my decisions on what to buy. Still it's never an impulse because that's how you get burned or end up with buyers remorse.