There was a JIM #51 CGC 8.5 that sold on ebay for $350-400 range and then was immediately resold for $1150 or something like that (at auction, I think) a few weeks later... Seems like this was about 5 -7 years ago that that happened. Someone can probably find out specifics on that book. I think the resale was off ebay. #51 is a robot cover - crossover appeal? I like the cover on 52 too, so maybe that's not a good yard stick. 54 is tough - a half dozen total copies graded?? But #56 is pretty tough too. What venue was the #56 for sale at?
Also, if you really like them, and a half dozen or dozen books come up per year from your want list and you're saving up - say $200 a month for a book. This is how your auction schedule goes: bid, click, dang - got outbid... next time through, a few hundred bucks more ready... bid, click, dang.... then you mentally add a few hundred more to the pile each time until you finally win one. Or on the flip side, you tune out for a few auctions after 6 losing auctions in a row.
I also think another thing that contributes to sale price fluctuation on the less common books is the venue. I think that there are more than just a few collectors out there willing to pay high prices, but frequently many of them just don't tune in to some auctions, or they don't have time to do a thorough search, or don't tune back in at closing time.
I completely missed the auction yet again, I had been watching two (non JIM) books and was frustrated to have missed closing time, only to see the post closing summary that they went well beyond what I would have been willing to pay...