The most I have spent on one single card is when I bought a 2012 bowman chrome xander bogaerts auto. It cost me $109.99, it is off to Beckett as I speak to get graded.
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The most I've ever spent on a single card was for my dream card, a 1933 Goudey Babe Ruth #144, that I later got graded a BVG 1. I bought it raw on eBay about 5 years ago, and grand total with shipping and grading fees I'm into it for $683.65. The last BVG 1 sold for $855 and I still have mine. I figure they will only appreciate, but for me that is my favorite card image of all time.
Thanks,
Jeremy