Summarize this article:

The simplest way to locate previous cost data is with your own memory. Perhaps there is some item at the grocery store or shopping mall that you've been buying for years at the same price. This could be a staple of your weekly grocery shopping trips or a clothing item you buy regularly. For example, imagine that the price of a gallon of milk has been $2.50 for a number of years. This represents the previous price for the purpose of your cost increase calculation. If the price of an item you've purchased in the past has increased, you can now calculate a cost increase percentage for the item. However, you will need the item's current cost first. Check the store for the current price of the item in question. For example, imagine that a gallon of milk, that has always gone for $2.50, is now priced at $3.50. You can now calculate the cost increase percentage to know just how much you are being charged at the new price in relation to the old price. Before comparing, make sure both values (your previous and current values) refer to the same product. If one is better in some way, your costs are not directly comparable. In some cases, locating previous cost information is not as simple as remembering the former cost of an item. For example, when comparing costs from long ago to now or finding cost increases on an item you've never purchased you'll need to locate cost information elsewhere. The same is true for cost measures (rather than items), such as the consumer price index (CPI), a measure of average consumer prices in the U.S., or the purchasing power of the U.S. Dollar.  In these cases, you will need to do some online research to locate previous cost figures. Try typing in the item and year you are interested in, along with "cost" or "value," to locate cost information for that time. For example, cost information for various consumer goods from 1900 until the present can be found at http://mclib.info/reference/local-history-genealogy/historic-prices/. For all historical cost information you locate, you will also need a modern figure to compare it with. Try to locate the closest modern version of whatever item or measure you are comparing. Make sure to avoid comparing items that are different due to different levels of quality or additional features, for example. Use the most current information from the current year in your calculations.
Recall the previous price of an item. Check the item's current price. Research historical cost information. Locate modern cost figures.