I'm a freak when it comes to that set up. There are several threads about it. I'm probably going to re powder coat the whole thing at some point, but most of the parts you can still get, incl. the drawer liners for the 557. And all the aluminum drawer pulls.
I paid 600 for mine off the SO truck, and this included a side box. I think a 274 or 273.
I've seen similar sets up on CL for about $400-$500 but I wasn't sure if the bottom box was a 557 or not.
I'd say $800 above, isn't unreasonable, but maybe you could do better in this market, depending upon your location.
Whatever happens, I've found that you're putting in a lot of time if the box has some problems (bent interior sheet metal, replacement of false channel etc). Though it's fun (for me at any rate) to restore the few that I've gotten.