Football card values are decided the same way every other card's are: by what one like it just sold for. What is different about football is the shape of the market. Demand concentrates hard on quarterbacks and on rookie-year cards, it moves with the season in a way baseball's does not, and the modern parallel structures are dense enough that two cards which look identical are often not.
Identify the exact card first
Modern football sets carry long ladders of parallels: the same photograph and the same number appear as a base card, as several coloured variants, and as serial-numbered versions running down to single digits. The differences are often a border colour or a foil pattern. Before you look up a price, work out exactly which version you have, because the sales you find for the base card say nothing about the numbered one.
Rookie year is the pivot
Football collecting is organised around the rookie card more heavily than most sports, and a player's first-year cards carry a premium that their later cards mostly do not. That premium is not a formula you can apply; it shows up in the sold prices, which is where you should be reading it. If you are comparing a rookie card to a later card of the same player, you are comparing two different markets.
The season moves the price
A football card's price is more seasonal than a baseball card's, because the sport is. Interest rises through autumn and around the postseason and falls in the spring, and a player having a run of good games moves his cards inside a week. That is one more reason to keep your comparables recent: a sale from eight months ago may have been made in a different market.
Then take five sales through the checker
With five completed sales of the exact card in the same condition, the card value checker gives you the middle three averaged, the mean for comparison, and the spread between the high and the low. If the spread is wide, the usual cause in football is that one of your five is a different parallel, and it is worth going back to check.
Questions people ask about football card values
Are quarterback cards always worth more?
They usually trade higher than cards of players at other positions from the same set, because demand is concentrated there, but that is a tendency in the sold prices rather than a rule to apply. Read the sales for the card in front of you.
What about cards from the junk wax era?
Football had the same overproduction problem as baseball in the late 1980s and early 1990s. Base cards from those years are common in good condition and are usually worth very little, however well known the player is.
Does a card being graded change what I should compare to?
Yes, completely. A graded card and the same card raw sell into different markets at different prices, so compare graded sales to graded sales at the same grade, and raw to raw.
How do I value a card nobody is selling?
You cannot, honestly. If there are no recent completed sales, there is no price to read, and the useful conclusion is that the card will take time and patience to sell rather than that it is worth a particular number.