If you search a few gun auction websites, it will give you a general idea of their retail value. The straight sale price will be a little lower.

You'll need to know the actual model of Mauser that you have to get accurate info.

Another idea would be to convert these rifles into something youactually want or will use. Any good gunsmith can rework them into a semi-custom rifle for you.Add a new stock and barrel chambered in caliber you like.