The most I've paid for a vintage piece was $42 for a Griswold No. 10, pop over pan, variation 10. It's pretty rusty but no pitting that I can see so far. It's waiting for warmer weather so I can set up an outdoor electro tank.
The absolute most I've paid is $56 (retail) for a brand new Lodge Logic 12" camp oven a couple years ago, and that was a pretty good deal even then but not what I'd call a steal.
The best deals I've made so far were the single notch #3 Lodge skillet I got for $12.50 and the #3 Wagner pie logo skillet for (IIRC) $8.
I find stuff in the $2-$3 range often enough but it's usually pretty bad, sometimes even toast.
My mother-in-law gave me a Wagner 1060 to replace the skillet I broke. She picked it up at a yard sale and I'm pretty sure she didn't give more than $5 for it. $5 is her limit for anything at garage/yard sales. She also gave me a #6 Griswold small logo with grooved handle that she picked up at another yard sale ... $2. This is the piece that got me interested in collecting.