Cost is the term used to refer to how expensive something is, most often used in reference to Gold paid out for equipment and supplies.
Some items, generally story-important weapons, have no cost, as they can be neither bought, nor sold. However, anything that can be purchased has a cost. Most items/weapons can be sold for roughly half their current value/worth, but in some cases items/weapons can only be sold at 1/4 of their value. Note that there is a difference between the amount something is worth and the amount that it sells for.
While a Silver Card reduces the cost of buying items by half, each item bought with it is considered to be its full worth for the purpose of the fund ranking in Fire Emblem: Rekka no Ken. When buying weapons, the more powerful a weapon is, the more expensive it is. Special items are also quite expensive, too.