Sounds like you've either got the caliber, or model wrong. If it's a .25-20, then it should be a model 1894. If by chance it's a model 1893, then it's most likely a .25-36 Marlin, not a .25-20. They didn't make the model 1893 in .25-20.
Values vary so much that it's nearly impossible to determine even a "ballpark" value on any old gun without knowing everything about it. Too many special order features, and condition to consider in determining values. Close examination is the only sure fire way to know. Great pictures can get a ballpark estimate, but nothing more.
Marlin lever actions 1870's-WWI, Ballards, and single shot rifles!