I feel the author/creator of this game needs to make sure it is CLEAR to potential purchasers that this is a DEMO.
Also specify whether the price will increase over time (as more content is complete) or if this is just paying to play earlier.
Alternatively, just call it Early Access. People seem to respond better to that phrasing in many cases.
Personally I wouldn't sell a game 'demo' until it is at least 75% 'content complete' but that is the sole choice of the developer of the game as to how they want to do things.