First, it depends completely on the specific product and what you're looking for. Some $100 cans sound great, some don't. Same goes for earbuds. Earbuds are great for portability and traveling, as they'll isolate you from the noises around you and are generally easier to drive than their larger counterparts. High quality cans are the "audiophile" headphone if you will. You can find some very good deals but generally speaking, you get what you pay for and the law of diminishing returns is in full force, just as it is in computer hardware.
It depends on the person and again, the equipment, but generally $200 headphones will have the potential to sound much better than $50 headphones. One exception could be for portable audio however. An expensive, hard to drive pair of cans may sound worse in some cases than a cheaper, easier to drive pair if connected to a portable device.
What are you generally going to be using these for? I think that's a good starting point.