I've been doing analysis of comic values for some friends who have some old issues coming into their hands from their recently deceased parents or other elderly relatives.
1) You' can't depend on Overstreet (I know a lot of you are saying "Duh!"). The guide numbers are largely supplied by dealers and they are not likely to give huge decreases to any book. Yes, in a funny way, you can determine some things....if the guide shows flat, it's probably declining. If the guide shows decreasing slightly, then the thing is in free fall over the years.
2) As people state, there are keys. The problem is, some of those are only known after the demand becomes visible and grows. I bought GSX 1 and IH 181 off the stands...I didn't think they were anything special at the time... but it became clear later when, at a comic sale, I was asked specifically about these issues. (The GSX1 I then calmly walked to my 50 cent box and grabbed out for the person who was breathlessly looking for it...that was the first I heard it was an in demand book. This would have been 1981 or slightly later, so it took 6 years). Since that time, though, they have been sought after continually.
3) IH 181 has been relatively flat in recent years...but now notice that even MVS missing issues are getting decent purchase prices.... They are definitely getting some bump from this bear comic market, but I think this shows a definite collectible demand.
4) There are past characters that can be cautionary tales. Take L'il Abner for example. This was an incredibly popular character, spawning a movie, stage play, and even the concept of Sadie Hawkins day. Where is he now? The comics had a build up a flattening and decline. Another to look at is Charlie McCarthy. Deadpool might be today's Charlie McCarthy.
5) pop culture awareness is key to the continuing value. Superman, Batman...and Wonder Woman. The only continually published characters... AF15 was always lagging FF1..Prior to 2000, you all would be listing FF1 in these recommendations...until the movies. Dells are stagnant/falling, but were once very popular and garnered increasing prices. Picture covers of John Wayne or Annette Funicello don't mean anything to anyone born after 1965.
(Can anyone answer the great mystery of why Moon Knight intro still garners large prices???)
6) IF you are going to invest...try to buy something that is obviously in beautiful shape, or 9.8 if slabbed (or higher). If it goes up, that specific item will go up the most.
So..now, you are going to gamble 800 on something? This is what I would do....first, mentally prepare yourself that it might be a total dump in the toilet of your money... Once that is done...find something that doesn't have huge public awareness now but you are betting will, or that will be looked on in 20 more years as super key books.
How about Marvel Super-Heroes #13? How about Swamp Thing #21? A set of Dark Knight Returns? If Dark Justice League pops, if the Sandman movie and Preacher are decent, Vertigo may be looked back on as a golden age of mature comics, and Swamp Thing 21 can be argued as the start of that.
P.S. Don't forget original art.