What can I expect to be offered for $500 - 800? or am I low balling it too much?

You have options, a used Beretta 686 essential or some of their other budget models or a used Citori (one of the Japanese barreled versions), they will be on the upper end of your budget. These guns were made to do go 200K plus rounds without breakage and you can often find used safe queens that have seen maybe 100 rounds down the barrel for 75% of the cost.  Some people don't like Beretta's, typically if they are bigger guys with my experience.  Larger guys seem to prefer Browning's.  You can find SKB's in the that price range. I don't like the square receiver but you might not have an issue with it.  

I avoid Turkish steel like the plague, the once exception may be CZ.  I personally would not spend money on one but if you only take the gun out a few times a year that might be an option for you, same with Franchi.  Again I don't recommend them but for light use they may be a cautionary option.

Too many horror stories about Stoeger, Mossbergs, Tristar, etc... to even consider those guns.

Ruger Red Label may be another option.  I have one and don't really like it.  Heavy, not fun to carry in the field, more recoil, and trigger slap really sucks on the 12 GA's.  Some people really enjoy theirs, I did not enjoy mine.

I have a CZ canvasback. I only missed 4 days of waterfowl season last year if that tells you how hard I treat it. It has never failed me and still looks good! I also have the bobwhite which is the side by side and have had the same experience with it.