i've seen steel ones go for about $100 used.
i sold a brand-spankin new aluminum one (for '02+ models) for about $250 last summer.
easy way to tell if you're only looking at pics is that the holes in the front 1/2 of the swingarm that the shock mounts to and the rear cylinder's header passes through will look smooth/rounded on an aluminum unit cuz the front 1/2 is cast. on a steel unit, that area looks more boxed/industrial cuz it's all welded box tubing.
i also like that both of those swingarms have spools (for a rear stand) built in. good stuff. the rear stand i use has "pads" that slip under the swingarm which is plenty secure, but can obstruct rear wheel rotation if i don't place it just right.