The bayonet looks to be a fine unmolested example of a "Transitional" 1898/05 aA Sawback bayonet. Transitional meaning that the early muzzle ears have been retained and the flash guard added. The early leather scabbard is also a nice example. The unit marking belongs to the 20th Field Artillery Regiment. Wartime bayonets with unit markings are most desirable and not often encountered. I'd go for it! It would be even better if the scabbard had a matching regimental.
I apologize regarding the unit marking. Not sure if it is a 30, or an 80. The 4 would denote the company and the 17 for the weapon number. Value is whatever you feel comfortable with.