Every storefront shows the same number: the percentage off the recommended retail price. It is the least informative figure available.
What the percentage actually measures
It compares today's price to a price the game may not have sold at in years. A title that has been heavily discounted in every seasonal sale for two years shows a large percentage and is not a deal — it is the normal price wearing a badge.
Meanwhile a game that rarely discounts at all can show a modest percentage and represent the best price anyone has seen.
The question worth asking
Not "how far below RRP is this" but "is this unusual for this game".
Three things answer that: how close the price is to the lowest we have ever recorded, how long ago it was last that cheap, and how often it discounts at all.
A game sitting at its historic low that reaches that low twice a year is a different proposition from one that has never been cheaper.
The seasonal pattern
Most games follow a recognisable curve: a token discount within a few months, something meaningful at the first big seasonal sale, and a floor about eighteen months in that rarely moves afterwards.
What we do about it
We record the best price for every tracked game daily and score each discount against that game's own history rather than against its RRP. A deal at or near its recorded low is marked as such. One that is well above it says so, even when the percentage is large.
