I paid $450 for one last summer. Excellent gun and excellent choice.
A real M590A1 is a tough find right now. Think I heard Mossberg builds them in batches so they're hard to get.
The mil-spec gun has a metal trigger guard, safety button, and the heavy walled barrel w/ ghosts.
It will NOT have a ventilated barrel shroud. The factory shround won't fit on the heavy barrel.
The standard 590 has plastic trigger guard, safety button, and standard barrel. Can be had with ghosts or bead.
Either one is a fine choice.
Don't forget the bayonet!