I'll jump on the Mora bandwagon. The karda is usually quite small, perhaps 2-3 inches. While you can do plenty with a blade that size, I would want a 4-5 inch companion blade myself. Pretty much any Mora will do, but I'm rather fond of the Mora Bushcraft. It's slightly larger and thicker than most Moras, and has their great Sandvik stainless so you don't need to worry so much about maintenance. Still under $50. They also make a "Bushcraft Survival" model, which is the same knife with a ferro rod strapped to the sheath if you're into that sort of thing.
Edit: Fiddleback Forge makes really excellent knives, but they're way out of your $50 price range. Mora makes the best budget puukkos that I know of.