2018 MARKET UPDATE: NEW ENTRY ON THE 100 HOT COMICS LIST!
Never a hard issue to love, the first appearance of Thor is definitely worth a place on the Hot 100 following a couple of years when its values had been static.
The Marvel key issue comics from the 1960s have long had their "first appearance" movie values baked in. That said, books like Fantastic Four #1 and Daredevil #1 have been considered under-valued lately, and investors have been taking notice.
Some recent sales:
You can see from the above prices that price compression has been occurring -- this is where two grades (or even three grades) of the same book sell for similar prices in the market.
This can be a fluke, but you definitely want to be the person buying the (say) CGC 7.5 for $15K rather than the CGC 6.5 for $13K.
Be patient. Don't rush, especially if you're investing five figures in to a Marvel key issue comic with the hope of getting a decent return on that investment in the future.
Some books just get more expensive suddenly. It's like you reach base camp on the previous grade, and the next step up is steep.
This happens to be the case with JIM #83. A CGC 6.0 is about twice the price of a 5.5, and grades above 6.0 get silly.
You can find a 5.5 that looks just as good as a 6.0 -- sometimes better. Try to avoid Marvel chipping, and look for a copy with most of the damage on the back cover.
Note that when you come to sell, finding a buyer for a book in the $5,000-$10,000 range is going to be easier than finding one in the $10,000-$20,000 range.