Because people want antique firearms (not papered if you will), usually a small premium on those as they can be bought and sold without dealing with FFL's. As AlexanderA said, it is a bit tricky on 1898's and depends on the date of manufacture (determined by serial). Springfield Research Service can give you a letter if you wish but you will pay for it.
At $600, you are getting it at the price of the parts if you chose to sell them. Good call. Go ahead and enjoy it as it won't ever be a pristine 99% rifle again but be very careful of the stock. Depending on your aptitude, you might want to dismount the action and thoroughly reoil the stock with boiled linseed oil. The stocks on these are old and can be brittle and the Krag stock is nowhere near as durable as Mauser or Springfield stocks. Reoiling it helps strengthen the stock.
Be very careful about removing the handguard as these are thin, have metal prongs for retention that can rust weld to the barrel, and you must raise (or preferably remove) the rear sight and maneuver the handguard opening around the upraised sight ladder. The reason for the handguard shortage is they had different models for different sights and they cracked quite a bit either from improper removal or from wear and tear. If not kept oiled, that makes for a bad combination of thin wood and brittleness.
Alexander A is correct on the sights. Probably the best of the lot imho if you like to fire it is the 1901 Buffington sight which is adjustable in windage (right to left). It resembles the original Springfield Trapdoor sight (which is was modeled after it) and the early 1903 sight (not the rod bayonet model 03 which used a similar sight to the 1902 Krag) was modeled in turn on the 1901 Krag sight. The 1901 and 1902 sights are superior because the front sight is completely fixed and so Kentucky windage had to be applied to 1894 and 1896 sights. Can't remember about the 1898 sights but I do not remember such as windage adjustable.
Complete examples of the sights alone go for about $100 or more and the carbine version (marked with a c on the sight ladder) about double if you could ever find a complete one. The 1902 sight is the 1898 sight with a small flip aperture sight on the rear of the sight blade. Resembles the old original windage adjustable SMLE sight more than anything with that ap sight. Those are also collectible but I find the 1902 aperture much to small except to really young eyes. On the handguards, Alex A is again correct and each sight type takes a different handguard. S&S Firearms has the aftermarket handguards on occasion and this guy is cited by some
http://www.partsforantiqueguns.com/Krag (Springfield) Upper Handguards.htm
The tiny screws on the sights are generally made of unreplacium as they use obsolete thread patterns which is especially important for the sight mount and things like the tiny leaf screw for the 1902 ap. leaf. You pretty much have to get original screws or tap the screw holes for modern threading as the CMP did for its new replacement Krag barrels--these are tapped for modern threads and screws.
A stockmaker,
https://www.shop.macongunstocks.com/Krag-Jorgenson_c21.htm, Macon Gunstocks is one of the few still making Krag repro gunstocks and I think they do handguards too. The cheapest is the generic handguard without inletting where you have to fit and inlet the handguard to the sights that you are using.
