Free: Contests & Raffles.
The difference between a $500 O/U and a $1200+ O/U is the precision and finish. Both guns will go bang and both guns will drop birds, assuming you can hit them. Like someone already said, the fit is of the utmost importance.
You need to go someplace where you can shoot a variety of O/U's and find one that you like, and that fits you well. There are some very expensive O/U shotguns that I've shot, and quite frankly wouldn't purchase if they offered them to me at half price. Do not buy an O/U relying on others advice or reviews, get out there and try a couple. With O/U's it really comes down to personal preference and fit IMO.
I have a Beretta S686 Special that I bought used in 1982. (Mentioned in the "My First Gun" thread. It's the one they decided was a classic, so they revived the 686 series recently.) It's fired 10's of 1000's of rounds and it's never failed! As close to flawless as any gun I've ever owned.