Cardboard boxes with any staples removed, a car or wheelbarrow tire, plastic water bottles with caps removed, a piece of rope with knots tied at the ends (great tug toy), an old t-shirt with a knot tied in the middle, an old heavy duty sock stuffed with other old socks and tied at the end.....those are a few things that come to mind.
Dogs don't care how much you spend, they care about how much fun the toy is, LOL...and much of the time, those "found" items are more fun than anything purchased!