Runner's World did
this story on affordable gear. They don't have a lot of good things to say about inexpensive shoes, but they did find a pair for $45 that made the grade.
Good shoes, properly fitted, are really important for avoiding injury. I hear you on the budget, but consider at least checking a specialty running store in your area (if you have one) to learn about how shoes can help with pronation, etc. At a good store they will watch you walk/run, examine your old shoes, and be knowledgeable about what shoes fit different types of feet (wide, high arches, etc). They might have a deal or at least help you find a brand that fits you well, so you can search for a deal online.
Good luck!