Difficult to say the value, especially without pictures of the exact rifle, but $400 seems a good deal if it is original and matching.
Danzig, Amberg and Erfurt made similar types of sporters in the aftermath of WWI, Amberg's and Erfurt's are much more scarce than Danzig's. Probably because Danzig made them longer (it was not restricted by Versailles, it was a semi-independent city state, under the protection of the League). Serial ranges suggest Erfurt made nearly as many (difficult to know how many were made, the serial is under the stock and most do not take pictures of the serial, but serial ranges approach the Danzig range. Far fewer are known though), but fewer have survived. Amberg seemingly made very few, possibly only 1000, maybe less.
Danzig made two types, one based upon the Kar.98a, which this is one, and another a .22 model. Both for export it seems, as most, if not all, are marked "GERMANY". The .22 are much more elusive, the 98a variation were made in large number, at least 8000 by serial extension. They are not rare in excellent condition, but $400 seems a good price for one.
Post up the serial number if you have it.