Malibu make a "Jump Seat" and IIRC it runs about $250. I am in the process of making my own. I have already built the frame, and I just need to carpet it. I am having someone else make the cushins, I will just be attaching them once they are finished. Total cost *should* be around $100!
i think you could make them cheaper then that if you do the vinyl work yourself. You would only need about 2sq yards of vinyl, some foam padding, some plywood, 2x4s, plywood, and carpet.