Here's another vote for the Beretta 390. (Surprised to see Eddie on the 390 bandwagon. I thought Brits hated any repeating gun with a single barrel.)
Like Carl, I got mine at Walmart for just over $500. I think they are $550 now, if your Walmart still carries guns. In Maine at least, they are not available anywhere else, although the Beretta 3901 is available from any Beretta dealer. It has a few more bells and whistles, but is otherwise identical, and I think is around $800 in the black plastic version.
My duck guns get exposed to a lot mud and salt, and I just can't see spending any more money than I have to. I like a semi, and the 390 has been almost flawless. Make sure you take the stock apart and clean the recoil tube and spring every once in a while. The very few times mine has failed to cycle it was because there was dirt and moisture in there on a cold day, and ice stopped the gun from cycling well.
.