Given the condition, (finish wear, 'stake' mark on ramp, maybe bent windage screw), I'd estimate value of your sight at about $150 to $175. As Whig said, buyers are limited.

It's kind of comparing Apples and Oranges.

The 1898 rear-sights sat lower and used a 'flatter' shooting (2,200 fps) cartridge, (with a 220 grain projectile).

The 1901 and 1902 sights, that replaced it, sat taller and were 'ramped' and calibrated for the trajectory of the slower 2,000 fps loading, (also using a 220 grain projectile).

Repetition is a key to real learning!

FWIW:
Occasionally, you see a Krag, that has worn a model 1898 sight for a long long time. Maybe some rifles and carbines escaped being updated.

(Another possibility, is that intact obsolete model 1898 sights may have been re-used to replace broken sights, during National Guard use or WW1 preparations, when the Krag was a reserve weapon)?

The top part of many model 1898 sights was visibly altered, (corners ground off, and serrations added to leaf), and coupled with new bases and slides to make more model 1902 sights.

I don't believe the model 1898 bases were officially intended to be reused. The curve of the base(s) was wrong for the trajectory of 220 grain, 2,000 fps ammo. The spring for the leaf was totally different from that of the model 1902 base.

(The obsolete bases were a surplus item that were long reused by surplus arms dealers and hobbyists to make unofficial sights).

In the Pre-WW1 era, ammunition became available loaded with 180 grain pointed projectiles moving at a velocity faster than 2,000 fps. This ammunition saw some use in period 'Shooting Matches'.
I don't imagine any Krag sight model was specifically calibrated for this ballistically superior ammo.
Shooters very likely noted the sight-setting necessary for shooting at standard distances and recorded it in their 'score books', for future matches.

IMHO - The 'three-notch' model 1898 sight could serve as a useful sight for a shooter. The correct settings for various distances would have to be learned by trial and error.