100% original "Imperial" Kar98a are much less common than those that stayed in German hands after 1918, but surprisingly they do not command prices one would expect these days. BiO just bought a rather sweet “Imperial” 1918 Erfurt on the cheap, and it had languished for months on Gun Broker before he took a chance on it.
Many "Imperial" Kar98a encountered today are sporter jobs, I actually have a couple I use today, really nice rifles but chopped stocks- they make incredibly light and handy shooters and I can see why they did this back in the day.
Impossible to say the value of your rifle without pictures, or even if it is "Imperial" as sometimes the difference is subtle, - a bright bolt and the follower is the easiest method, as in almost all cases if the 98a stayed in German hands it will have a notched follower to stop the bolt (Imperial the follower will not stop the bolt), and usually post-1918 will have a blued bolt.
That said, of the wartime Kar98a, 1915 is fairly tough to find (quality "Imperial"), and each year they become slightly easier to find (1915 is harder than 1916, 1916 is harder to find than a 1917 etc...). When I was on the Kar98a kick, before I got hooked on Simson, and now Loewe, I had a devil of a time finding a nice 1915 Erfurt 98a, and never found one I was really thrilled with.