There are plenty of very different choices out there. My advice would be to make a list of your priorities first. If you're used to your Opinel, you'll actually have a hard time finding a pocket knife in any price range that cuts so well and so efficiently. It's all got to do with the Opinel's thin blade and acute edge geometry. Many folders are stronger or more heavy-duty but few come close in terms of sheer cutting or slicing ability.

For $50 you can get a seriously hardcore folder. I personally EDC a waved Endura, which is a pretty large but still light and slim knife. Spyderco has a lot to offer in the $50 range. The design is usually weird and takes some getting used to but at the end of the day their knives tend to be very functional, well made and the blade steel and heat treatment are usually top notch.