If it's an 056 standard in really, really good shape, you could pay $175-$200. The reason to buy a standard is that you can put an 056 Super jug and piston on it and have a whole other class of saw. For good Supers you'll see them go for $250 or so in good shape, and for the Mag II, which is the way to go in my opinion, can go fro $300-$500 depending on condition. Condition is everything, and how well it's been taken care of. I sold a Super in prime condition for $450 last summer. And then a couple of months ago I bought a sound, running Mag II for just over $200 because it was missing a muffler.
The 056 is a great saw, and is a favorite amongst timber guys from back in the day. It is heavy, but there's a romance to a big heavy, slow revving saw like the 056.
Jeff