Any fixed blade hunting knife will do - preferably the simple drop point, about 4 inches. If its over $40 dollars, you'll have money left over for other more important gear, like water purification equipment. Soaking $100 or more in a knife that sits unused in a bag all day is a gratuitious toy, not a tool. And if the bag is stolen, confiscated, or accidently forgotten somewhere, it's not a huge loss. It's also not a major indicator of some evil intent when viewed by an LEO - unlike most mall ninja toys packed with 1% solution gear in a tactical callout bag dragged around by 5.11 wearing wannabees.
Keep your bugout stuff simple, and you will have good, simple tools to use with the level of your skill.