The best made O/U - (some of which can easily go well over 50,000, let alone 5,000) are hand finished by highly skilled craftsmen to customer specification. The ones around 5,000 are of very good quality but have more machine-done work. Their barrels are well-regulated, the guns are balanced well; the POI/POA is the same, the triggers are clean, crisp, with no creep. They are designed to shoot a LOT of shells before needing repair.
The $500 guns have none of those features. While they may appear close, upon a thorough inspection the wood to metal fit is typically flawed, the gun has very poor balance, the triggers are usually heavy, lots of creep and may have a gritty feel. Their barrels can be suspect when it comes to POI/POA regulation. On one forum a person just bought a Huglu-made CZ and on one of its very first shots, he had issues and had to send it back to CZ for repair