Pricing is obviously an important topic to most collectors - nobody wants to overpay for something, and rare, sought-after comic books can be quite expensive. Today's collecting marketplace is also very speculation driven - prices to books that are designated "keys" can rise rapidly, and while nobody wants to be left behind, it's also important not to get carried away by Fear of Missing Out.
The "market price" is obviously a moving target, and is really only dictated by what somebody is willing to pay for something today.
That said, historical price guides can be extremely useful in terms of benchmarking what you should be paying for something, and looking at developing trends; however, the quality of information can be highly variable - not just between platforms, but it also changes with time, depending on who is collating the information. Different platforms source their information from different places, and sometimes make mistakes.
As an example, one platform recently noted a number of $85 dollar sales on the 2nd printing of Black Panther #3 (2021), which drove up their estimation of the average price. However, if you were to go into the data, and then track down those sales on Ebay, you'd find that the sales were also for sets of the 2nd printing as well as the 1st printing. As the 1st printing was going for some $70, that would suggest the market price for the 2nd print was actually more like $15 (depending on how you want to interpret it as part a set sale).
So if you're looking to spend a reasonable amount of cash, it's worth digging into the data a bit before making a decision.
We like Covrprice, primarily for its interface, which is good to use, and visually appealing; we would also recommend GPA, which has a good reputation for the quality of its data, and tends to be the go-to platform for dealers. They are reasonably priced; as we maintain our personal collections in Covrprice we have annual subscriptions, and pay of GPA on a monthly basis whenever we need it. If we're researching something, Covrprice would be our first port of call, and if it's a pricey book, we'd cross-reference GPA. Then we'd go to the main source of information - which is the Sold Items section of Ebay.
Inside of that triangle of information is where we'd position our assessment of the market price - obviously taking into account any outliers the data sets have captured, outliers, anything like signatures that might have had an impact, etc. Some degree of interpretation is going to be necessary, and the degree to which you go into everything is obviously a function of how much time you've got to spend on this, and how much money may be at stake!
Even then, the market may surprise you: just because you've done your research, and determined a reasonable price range for a book, doesn't mean that somebody else with more money isn't going to bid over the odds for something they really want.
That may happen particularly with rare or expensive books - as the market becomes more illiquid as you go up in price. For example, if you were looking at the rare variant of Clone Wars #1 (on our long term want list, for sure), there are relatively few people who are willing and motivated to spend the kind of money it goes for, compared to a more affordable book. If those people happen to be busy with other stuff when the book comes up for auction, you might get it for a good price; but if they're all excited for the Ashoka show because set photos have come out, the price may well take a jump...
Other sources of pricing information that you may want to look at include GoCollect; we would be wary of the pricing section of Key Collector which isn't updated regularly, and as a result is often out of date (which isn't to say that Key Collector can't be useful, we'll come back to that in a subsequent post).
The Overstreet Price Guide is basically useless at this point. There was a time - primarily the 70s and 80s - when it was useful, because the market moved slower, so an annual publication could give an indication of prices and not be all that far off; but today's market moves so rapidly (both up and down) that a printed publication is going to be out of date before it comes off the presses. Websites and apps can update their pricing multiple times during a day, and a printed publication just can't keep up with that.