there are some very nice shotguns out there in and around your price range.
what i would like to know
-how old are you
-what is it to be used for "i.e- upland birds,sporting clays, turkey"
now if you plan on shooting a whole lot and don't want to spend much on ammo the 12 is by far the best choice. however a 20 or a 28 carry much better in the woods. you can get the same 12 gauge gun in a 20 and it will weigh at least a pound less.
i own and have shot almost every gauge out there and i will not lie if i had to choose one for the hunting i do i would take my 28. i hunt pheasants, woodcock, crows and some squirrel with shotguns and i have had great success shooting winchester high brass #6 shot out to a good 40 yards. however the ammo isn't cheap but i believe it is worth every nickel. i guess i am a little bias [:-]
as far as what action "pump, semi" which ever you like better. however if you don't have a long arm reach and are shooting a large 12 gauge pump you may have problems with the action
also if you want to look at side by sides EAA makes respectable ones under 400. i have one in a 12 and it works great
give us some info and let us help you