The first question is whether that horn is stamped "high pitch" or "low pitch."  If it is a high pitch horn, its main use is as a decoration. 

That horn is a "stencil" horn. Stencil horns were inexpensive horns produced for merchandisers who sold them under their own brands. They were often good horns. The features were often slightly different from those on the horns that the manufacturers sold under their own brands, and based on earlier designs. That horn has some features similar to early Buescher True Tones. 

The horn looks in reasonably good shape, but some of the keywork is antiquated even by vintage horn standards. It lacks a front F bridge key, which puts a severe limitation on the playability of the horn near the top of the range. The button style G# key is clunky to play. So if you're looking for a horn as a mainstream player, you might find those aspects of the keywork frustrating.    It might be a fun horn to learn on, but it will limit you to fairly rudimentary technique.

The fact that it's a stencil horn and not a manufacturer branded horn also affects its value and the price reflects that.  So.....is $300 reasonable for its condition and inherent limitations?  I think so, IF it is a low pitch.  Is it a great deal?  I doubt it.