That's an easy one, and at its core, something that hasn't and will not, ever change. The main thing that gives a comic book its value, is its contents. Not its age, not its character or title which is a massively common misconception, Sure, these factors have an influence on the value, but not as much as you may think.
The main thing that gives a comic book its value are key events. But what is a key event? Now this is where it gets harder. In a nutshell, the correct answer, is 'anything'. Technically, a key event should be, by definition, a key event - something pivotal to the story, something that changes the outcome of the plot, etc. etc. But with comic books, this has evolved beyond that.
A key issues encompasses not just this, but artist's firsts, writer's firsts, even phrases used by characters for the first time. Family first apps, such as black cat's sister, or even more tenuous like the green goblin's sisters aunt petunia (FYI not an actual character). Whatever the reason, if the book does not contain one, its value will not ever rise drastically.
Of course, to muddy the water further, there are exceptions to this rule. Missprints for one, e.g books that were meant to be printed a certain way, but a small number of them turned out another way. Recalled issues, books that were recalled for various reasons, IE elseworld 80 page giant by DC was recalled due to a baby superheroe crawling into a microwave and getting nuked,. Even low print runs can bump the value, there is just too much to cover.
Now, if you're new to collecting, and pricing up a book is alien to you, there are a number of tools that can help you along your way.
The two main ones:
This might sound stupid, but a comic book is only worth what someone is willing to pay for it. THIS IS LITERALLY WHAT EBAY SOLD IS. There is no need to purchase memberships to these US data crawlers with their massively innacurate values as they are based on the US market. If you're unfamiliar with eBay sold, all you have to do to access it is load up eBay, type the book you want valued in the search, click advanced, more options, and tick sold items. Done. You know have millions of sold item listings at your fingertips.
I cannot stress this enough. If you collect comics. Get Key collector. Key collector is the comic collectors bible, with listings that cover almost every comic book in existence (in the US market), and an intricate database documenting EVERY KEY ISSUE, with hundreds being added daily. ONE THING TO NOTE is that the values given on this app are not accurate, again they are scraped from US sales, not UK, though they are a fantastic indicator of ROUGH values, plus an easy way to check whole runs to see if there's anything of worth. Having said that, avoid the updates by 'Bry's comics' and the other price manipulative bullshit that they spew out now and then.
Simple right? < Sarcasm. Getting to grips with this stuff on a proffessional basis will take years of practice, with facsimilies, reprints, and even fakes being created all the time, it makes buying and selling at a collectors level quite the risky venture. But make sure that you take the time to research your purchases and sales, and you'll be safe.
Any questions whatsoever, join the Fantasy Road community discord server, which is a great tool in itself - an entire community of comic colectors at your disposal.
Hope to see you there!