There is no single baseball cards value, and any page that gives you one is quoting an editor rather than a market. What exists is a record of completed sales: what somebody actually paid for the same card, in the same condition, in the last few months. Everything useful follows from reading five of those properly, and the whole method fits on one page.
Sold, not asking
The first thing to get right is which number you are looking at. An asking price is a seller's hope and it can sit on a listing for years without anybody meeting it; a completed sale is a transaction. On every marketplace that publishes them, completed sales are a separate filter from active listings, and the gap between the two on ordinary cards is routinely large. If you take only one thing from this page, take that: read sold, never asking.
Same card, same condition, same holder
A comparable is only comparable if it matches. The player and the year are the easy part; what catches people out is the parallel, the serial numbering and the holder. A base card and a numbered refractor of the same player in the same set are different cards with different markets, and a card in a graded slab and the same card raw are priced as if they were different objects, because to a buyer they are. Filter hard before you write any prices down.
Five sales, and drop the two extremes
With five recent sales in front of you, ignore the highest and the lowest and average the three that are left. The high sale is usually two determined bidders in the same minute and the low one is usually a listing with a misspelled title that almost nobody saw. Dropping both is a standard way of describing where a small sample actually sits, and it is what the card value checker on this site does with the figures you type into it.
Read the spread before you trust the answer
The calculator prints the high, the low and the distance between them as a share of your figure. A tight spread means the market agrees with itself and the number is solid enough to act on. A wide one is not telling you the card is volatile; it is telling you that your five sales are not five sales of the same thing, and the right response is to go back and pick better comparables rather than to average more numbers together.
Questions people ask about baseball cards value
Is there one baseball cards value list I can look up?
No, and the lists that present themselves that way are editorial figures set on a publisher's schedule rather than records of sales. The only value a card has is what somebody paid for one like it recently, which is why the method here starts and ends with completed sales.
How recent do the sales have to be?
Recent enough that the market has not moved under you. A few months is usually fine for an ordinary card; for anything that has been in the news, or for a player having an unusual season, the last few weeks is the honest window.
What if I can only find two sales?
Then you have a thin market, and that is itself the answer. With two sales you cannot drop the extremes, so use them as a range rather than a figure and expect the card to take a while to sell at either end of it.
Does condition really change the number that much?
On anything that is not vanishingly rare, condition is most of the price. Corners, edges, surface and centring are what buyers are paying for once the card itself is common, and comparing a well-handled card to sales of a sharp one is the commonest way people end up disappointed.