Over the years the community as a whole has mostly destroyed all the components of these engines bit by bit with big single turbo and drift builds. As a result there is very little good quality usable stuff floating around at this point from which to put together blocks. At the time of this writing I have only a handful of components on hand, not enough to pre-built engines and send them out the door in this manner, only enough to replace 1 or 2 parts in a customer's core I may be rebuilding.
At this point you are pretty much down to having to buy new components one by one from mazda as needed, or spend the ~$6000 for one of those "all new parts" engine blocks that mazda periodically makes available for the REW/FD application (note that as of this writing, these are not available for other applications to my knowledge).
IF you buy the setup listed on my site, there is no core charge.
The topic of core return/exchange really only works for local/regional owners able to deliver their core in person to me in east TN.
Core value is determined based on what inside is still reusable, which cant be known without teardown and inspection. I can't negotiate for a core blindly and pay up front without knowing what's inside it, if it turns out to be a boat anchor then that's just money I tossed out the window.
Truck freight shipping, even at minimum for nearby/regional customers, costs the same as the value of a decent condition core, 3-400 bucks. I can't pay for both the shipping and the value of the core that the owner wants to sell me, so thats why it only works out for those who can deliver and wait while I tear down.
That's why I omit the entire discussion for pre-built engines listed on the website.