How much would I pay? depends.
If one is in the Chicagoland area, one of the best burgers in US is $3.80 at Paradise Pup, details here:
http://www.flyertalk.com/forum/showthread.php?t=450842
Another deal is the fois gras burger at Sweet & Savories in Chicago. I'll get a link sometime but basically a grilled Kobe beef burger, w/truffle mayo, slice of fois gras going for $12 on Wed.
so to answer the question, I don't believe I have a limit to a burger price (at least I haven't seen a burger price I wasn't willing to pay).
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