The most robust folder I ever used in the field was fashioned out of AUS-8A steel. Through many hunting excursions, the Kershaw Folding Field (KS 1050) gets very high marks in my book although the price point exceeds your parameters by a wide margin. That being said, you might consider using a fixed blade if you are intent on rough use. Folders by definition/design won't hold up as well as even a marginally designed fixed blade. I have a drawer full of slip joints and other folders that sport broken blades and bent tips that can testify to this assertion.