A K98 in original condition that was arsenal chambered in 30-06 would be worth between 400 and 700, depending on condition. Also it is almost certainly not an antique, as the cutoff for "antiques" is 1898 for lack of a better classification. If it is a Gew 98 that was refurbished into a k98 and then altered to 30-06 it would make it rather interesting, though not necessarily any more valuable.
If the alteration to 30-06 was done by a gunsmith away from the arsenal, the value decreases slightly depending on who's looking at it.
P.S. A good clear overall picture and clear pictures of any markings, especially on or around the reciever will get you a solid price estimate.