What, a complete 427 side oiler, or a complete XE engine ??
You can get a new complete side oiler from an engine builder for $14k up. For one built around a good original side oiler block this can be double, depending on spec. Mine is somewhere between the two prices.
As a guide, engine builders will pay up to $5k for a bare serviceable side oiler block on its own...
As for the XE engine, depends on whether it is serviceable / usable.
Based on the waterpump, if an Fe costs £20k and its pump is $200, that would make a complete XE engine - err - a hell of a lot !!