Well, if you look solely at the numbers, far more M91/30's were produced than K98k's. So on that basis alone, the latter would be more expensive due to supply and demand. On a more conjectural level, many K98k's were destroyed by the Allies during and immediately after their conquest of Germany. Also, the supply of K98k's after the war was spread out over a large number of countries, many of whom either trashed their rifles through use or just haven't given them up for sale to importers. Finally, the largest imports of the K98k into the US took place years ago, and those rifles have long since disappeared into private collections. So the available pool of these rifles is even smaller today.
The supply of today's M91/30's is much larger, many of these rifles having survived intact in the hands of the victors and their allies. Also, imports of large numbers of these rifles are more recent than those of the K98k's. So their prices will be a lot lower compared to K98k's, especially the non-Russian captures.
Beyond the numbers, the subject of perceived value becomes touchy and highly subjective.
