Ok now you say that the rifle is dated "1940" on the barrel? Is it on the barrel or on the front reciever ring? Is the rifle marked "Mod 98" on the left side of the reciever? Is there a code on the top of the front reciever ring (example; AR, BNZ, S/42, 42, S/243, ect.....)? Is the barrel 23 5/8" long? Does the stock run most of the length of the barrel and terminate in a metal "stud"?
Is there a wooden handguard on the top of barrel just forward of the rear sight? Is the wood laminated or walnut? Is the buttplate a half inch thick or is it thin? If you answer these questions then I can give you an accurate price. There are alot of variables that change the value of the rifle. If the rifle is original and unmolested and produced by a rare manufacturer or issued to a rare unit then it is possible that the rifle could be worth upwards of $4000. However if it is not 100% original, if the stock is shorted, if is a common manufacturer, or common issue it could be worth $100. I own over 200 mauser rifles, some of them cost me $20-$30 and others I've paid $7000 for.