Hi. When is the last time the shock was rebuilt? When is the last time it was charged with nitrogen? Ask yourself the same questions about the "$50 shock". I would guess that the shock could REALLY use a rebuild and fresh oil, plus nitrogen. They can't be run forever without maintenance. Unless you know the $50 shock has a fresh nitorogen charge on it, it may need nitrogen too. Even a brand new in the box 1986 KX80 shock will need a charge. I have heard people get quotes from free - $40 on nitrogen.
Sorry to not give you a honest answer that is inexpensive too, but that is usually how it goes on a 20 year old shock.