The trick is that the Chiefs Special is not an uncommon gun and there are few collectors who consider then to be their holy grail, unlike the registered magnum for example. So to most people it is a second hand carry gun, and S&W have a rebate on new 642s.This helps hold the price down.
In 2006 the Standard Catalog of S&W gave a value of $310 in very good condition. Allow a little inflation and $350 is about right. If it was mint with box then it would be worth more, $600 or so.
Now as for numbers, S&W stamped the serial number on the butt. There are usually assembly numbers,which were used to track parts in the factory, on the inside of the cylinder yoke cut out.
He may be quoting those. So the 36 may be an assembly number, as may the 20xxx. You need to verify this.