As long as it passes the rules you can use it. A racing bike is a bike that people would race on and that includes top and bottom end models.

That being said after travel, accommodations, incidentals, and entry fees the event is not particularly budget friendly which is why you may raise some eyebrows with a $25 low end bike. It is like paying for a $2000 tuition to a photography course and showing up with a 12 year old point and shoot.