A $5 to $10 Mora knife is a candidate for one of the best peformance to price ratios around. The relatively thin blades on the ones that I have, I have a couple of different Erikssons, a Frosts, and a few Eriksson blades, coupled with the hard RC60 carbon steel make for excellent cutting performance. One can get steel with better edge holding, and can certainly get fancier handles, sheaths, and such, but I haven't seen any fixed blades that come close for $10, excluding the discontnued Cold Steel butcher style knives and the Barteaux economy model machetes.
For a step up in a heavier duty knife the Anzas, made from annealed files, are nice with some models starting at $20 to $30. The 3in bladed #4 that I have takes a very nice edge. Another favorite in the same price range, that is unfortunately discontinued and evidently almost impossible to find in the US, is the Hubertus 4in carbon steel 'Bowie', although people in Germany called it a Scout knife or pathfinder. It has a stacked leather handle, a simple sheath, takes a very nice edge, and was 50DM.