This's probably already been suggested, but there're too many posts for me to want to sort through them all to find out for sure at the moment.
IMO, for maximum utility and toughness for the lowest buck it's hard to beat the old USGI issue item, usually reffered to as the "Pilot's Survival Knife".
Not especially pretty, and certainly not 'trick', but just as tough, versatile, and efficient for its purpose as it was back in the '50s when it was first issued. Comes with a decent leather sheath and a small sharpening stone in its own little built-in pouch. Usually found for about $30, brand new.
Beware of cheap Asian 'knock-offs'. The most common names to look for are Camilus, Ontario, and Kabar.