In response to user suggestions, PriceZombie is adding something new, and we think, unique. Our price history charts are very good at showing price variations over time, but users also wanted to know a product's average cost.
We calculate weighted average over time. That is, we consider the amount of time a product has remained at a price point, as well as the numerical value of the price. For example, if a product has a high price of $150 for 4 months and a $100 average for 2 months, the average is $133, not $125.
You can find the average price for an item on the product view and live price drops for 30, 90, 180, and 365 days. Look for the blue "info" icon, click it to get detailed averages. The browser extension shows a 90 day average. Information in the extension is limited due to space constraints but you can click on the link to our website, in the upper right of the browser extension window, to see the full range of averages for that item. If a product has not been listed long enough for an average to be calculated, it will be marked NA.