Yes they did used to be found cheap, but like so much vintage audio that people are now seeking, the rise has continued since 2013. $100/pair for Smaller Advents in good operational condition (even given the woofer's inevitable surround replacement) was a deal even back in 2012 when you consider what $100 would buy otherwise. I envy those that found these in thrift stores for next to nothing, but most people that want them will not have the luck to find them that way or will spend years waiting to do so, if at all.
That said, I paid $260 for SA's delivered 3-4 months ago on eBay. They came with the boxes in great condition, expertly refoamed woofers, and grills/cabinets in beautiful shape. Hard for me to believe someone let them go for that amount, but by today's standards count on spending $250/pair for SA's in good condition. Unless you're buying pre-owned speakers whose retail has plummeted over time, I don't believe there is anything at twice that price you can buy
new today that will provide more musical satisfaction, let alone be a proud owner of speakers with such historical clout. I would have paid $300 and still feel fortunate of the value (and enjoyment) I got.
To supplement the point in
my perceived value, I've a pair of Infinitesimal 0.2's in perfect condition including boxes and optional brackets. Love them because they sound fabulous with proper associated amplification, they're ultra cool, there's nothing on the planet like them (especially when using four of them), etc.. They would sell any day for twice what I paid for the SA's but the SA's are in about every imaginable way better sounding. They sell for far more because people want them for their uniqueness. In my opinion, if you like great sound and vintage audio, SA's
in great condition are absolutely worth $300/pair. Then sit back, listen, and feel great about how your hard-earned $300 got you far more than many speakers asking more could only hope to cleverly appeal to you in their classic '70's (and often fun) marketing literature!
As always, do research and protect yourselves from sellers that are not clear or obfuscate important details, which is the only way your dollars spent will have positive initial and lasting enjoyment.