As someone who's been doing this for only about a year, here is some advice I have, for what it's worth.
The generic advice to "buy what you like," is correct. But what does that mean? The only way to really know that is to look at a lot of art, and get a feel for what makes a great piece of original comic art. Some people only want Covers. Others prefer splashes. Some don't mind the panel pages. Covers are invariably more expensive to buy, expecially the older, pre-digital era pieces.
I actually think panel pages can be pretty great, as pop art. There is nothing like original comic book art, with the panels and word balloons. That's what makes the art special, and differentiates it from other types of art.
I bought my first piece over 25 years ago at a convention from Sal Buscema. At the time, he was not regarded as a 1st tier artists, so I got a great title splash from him for an absurdly cheap price, which was the going rate at the time.
Now, Sal has a much great reputation as a great Marvel stylist.
I didn't start buying any OA until more recently, with that piece as a starting point. I don't have any other higher end pieces...yet.
I prefer the older stuff, with the word balloons intact. Ink on Pencil. That's real Comic Book OA, IMHO. I also always collect the comic its from as well. It's especially fun when you come across a page from a comic you already own, and remember reading.
I learned "values," initially by just following eBay auctions. After a while, you get a feel for what things are worth, and can start predicting how much they will sell for.
I also set a budget on how much I am willing to pay for a piece. I've lost a few that I really liked because I was unwlling to pay more than my budget, but the great thing about this hobby is that there are hundreds of thousands of pages out there. You will find another one you like eventually.
I like 80's independent comic books stuff. And, because the books they come from are not so highly sought after, the pieces tend to be reasonable. Even from bigger name artists.
But, what I've found as a rule of thing on OA, is that the artistic merit of the piece counts the most. It's a visual medium, and things that just look really great tend to command higher process, no matter who the artist is.
I'll take a great splash page by a lesser regarded artist over a really crappy example page by a "star" any day. And, both do exist.
Second, well-known characters tend to be more sought after. Spider-Man and Batman pages, no matter who drew them, are going to command a premium, generally. Not always. But, generally. Especially if they are good examples of the characters in action.
Sexy woman art also brings a premium. A sexy drawing of any female in a published comic book page will tend to draw more interest, no matter who the character is. This is a male dominated hobby, so that's just a fact.
Superhero stuff tends to get higher prices than other stuff, unless it's from a premium artist, from a classic era, doing horror or sci-fi or humor.
Pages from well known artists Comic runs for a particular character, or from key issues, will command higher process than general run books by lesser known artists.
Grail pieces for one collector may not be so for others. Some may think a McFarlane spidey-splash or cover is a grail. Others, like me, would rather have ANY Dave Stephens page, especially if it's Rocketeer. I saw someone was able to score one of Stephens' classic Alien Worlds covers recently and was consumed with jealousy. But, after a while I thought how great that piece was an appreciated it.
In any case, the bottom line is - buy what you like, but don't pay more for it than you can afford. Quantity can also be quality, if you have a discerning eye and buy stuff you really like. There are a TON of great, great, great less expensive pages out there. They are just not from key books, or by superstar artists. They are just excellent examples of the artform.
I do advise going to conventions and just talking to artists there, and buy from them directly if you like their work. But, folks, real Comic Book OA is going the way of the dodo. In 5 years, comic books will no longer have original pencils or inked pages. It will be all digital. So, the stuff that exists out there now may be all there will ever be.
I am not a fan of the digital stuff. The finished artwork is great, don't get me wrong. But, it's not the same as real ink on pencil. That's just me, though. Others may - and do - disagree! That's what makes this hobby so great.