What is a core? A core is the old set of parts that is built into the fresh new transmission. Consumables are replaced, such as bushings, frictions, gaskets, filter, and seals (among other things). The parts that are reused are called hard parts. The hard parts used in a rebuild (with rare exceptions), are perfectly good used original parts that are either good the way they are, or sometimes various pieces can be machined as needed to be in acceptable condition for re-use. Over time, hard parts can be damaged through normal failures as well as being handled incorrectly. There are certain parts of certain transmissions that are harder to procure than others, or more expensive than others. The market determines what used parts are worth. If a core comes to me with obviously damaged parts, those need to be replaced before the transmission can be used again. This takes time and money. I have to be able to make use of what comes back, or the replacement cost of the necessary parts has to be taken into consideration.