A lot of parts are rotables and have a high core value for refurb.

Similar to going to the local auto parts store and buying an alternator or starter with a core charge. If you don't have your old one, you get charged for the core.

There are a lot of 727's still flyng internationally as well as cargo operations in the States that still make these parts expensive, but for the most part, not out of financial reach for a determined builder.