I am sure you mean .30-40, not .30-45. The Model 1898 carbine was standard issue for the cavalry, but they are uncommon since only 5002 were made before the Model 1898 carbine was superceded by the Model 1899. A Model 1898 in top original condition can bring $5000 or more.
But be very careful in buying ANY Krag carbine, a Model 1898 especially, since most of those on the market are cut-down rifles. If you are interested, get some of the good books on the Krag and study. And it would be better to be very skeptical of anything a seller tells you.
There is a good forum on
www.jouster.com that is dedicated specifically to the Krag; the experts who hang out there are always glad to help.
Jim