Only 169 Original pre-war Rigby .416s were ever produced on Mauser made Magnum actions.
They are collectors items and that's why they command the "ridiculous" prices.
The new Rigbys are made with actions made by Golmatic in Germany. The same action used by Mauser and stamped Mauser 98 Magnum, Made in Germany, on the left receiver wall. Mauser does not manufacture any 98 actions presently.