Every item in Dragon Age: Origins has, among its many properties, a "true value". This value is not seen in the normal in-game tooltip, you have to inspect the item from the Inventory. You can also look up an item's value here at the wiki on the item's infobox. The Diamond is shown here as an example.
The item's true value is used to calculate the selling price (how much money would a merchant pay - usually 25% of its value). It also serves to calculate the merchant's price or buying price (how much will they charge for it - usually its full value plus a 25%-100% markup). Every merchant has their own markup, which is the same for all items traded with them.
- Main article: Merchants (Origins)
While at a merchant's trading screen, a new field will be added to the item's tooltip, showing the selling price.
- Items on the merchant's side will show how much it will cost to buy (that is, item value + merchant's markup)
- Items on the party inventory will show the price the merchant will pay (usually 25% of value)
Items bought within the session (before closing the trading screen) will show the price the item was bought for, allowing you to sell it back for the same amount paid. Same for sold items, which can be bought back for the price they were sold for. After the trading screen is closed, the items sold to the trader will be priced at the usual full price + markup, and the items bought from him will only yield the selling price.