The price difference is very simple it's because they come from different suppliers and do not expect the next arrival to be priced so low.
Usually, the Zastava in metric calibers come from European distributors while the american or "standard" calibers usually transit in the USA before reaching us.
The low priced ones you see right now seems to originate from the USA (importer being USSG). My suspicion is that these are the left over of the Remington contract (bought back by USSG?). While this is good for us, consumers, it doesn't help to place the right value on these Mauser rifles.
When we consider that the average used sporting rifle made on a commercial M/98 worth between 350.00 $ to, say 600.00 $ for large production products, then, a brand new M/98 (add to this a Supreme-like action made of modern steel with a good quality adjustable trigger) at 450.00 $ is a real deal. Logically, the rifles should sell close to twice that price.