What will you be hunting? The answer could be different if it's quail over dogs or preserve type hunting, mainly dove hunting, pheasant hunting, wild birds, ect.. hopefully not ducks with a 20 gauge.
I assume you're considering a modern gun for "off the shelf" loads. If so, this could be helpful, a few years ago I literally went through just about every double under $500.00 , Russian, Brazilian, American (Stevens, Savage Fox, ect.), Spanish (Zabala, Richland, Sarasqueta, Laurona, AYA for Sears, Ect)., Japanese, (Westernfield/Miroku), Belgian (lucked in to that one, still have it.) .410, 20 ga, 16 ga,12 ga.
The first thing I would recommend, even though 16 is the "classic" gauge, is to get a 12 gauge. You can shoot 20 gauge loads (7/8oz) in the 12 and anything else you'll need for just about any situation.
The second is if you're seriously limited to around $500.00 there are still some good choices, in my experience and opinion mostly Spanish. (Except the chunky TriStar/Zabalas)
Here are a couple of good examples, that are available.
http://www.crosnoeguns.com/itemdetail.asp?ID=4361 http://www.crosnoeguns.com/itemdetail.asp?ID=4381If you like a pistol grip with a beavertail forearm, I enjoyed these type of Saraqueta's more than the other 'budget' Spanish guns and thought they had better attention to detail/quality.
http://www.gunbroker.com/Auction/ViewItem.asp?Item=134653891Though, I found 30" barrels was the best for my type of shooting and height.
If you're heart is set on a small gauge, look for a Westernfield by MIROKU, (not the Stevens Westernfield) in 20 gauge , this is a great gun that I wish I had back. They're a "needle" in a haystack kind of thing.
My situation and tastes have changed so most of what I shoot now is Damascus. But don't let someone's opinion about more expensive guns delay you from enjoying a decent low priced double. I'm sure even they would agree that almost about any double is better than a synthetic pump gun..
Doug