Only you can say whether a $100 is enough - certainly there tripods that will take a 40D for less than this amount, but you are usually compromising build quality, reliability, usability etc. so it comes down to where you priorities lie. Maybe you only need the 'pod for really low light, and you'd rather save for lenses.
I will say this however, there's nothing like a good tripod. The confidence that when you step away from the camera it's not going to fall over and smash into pieces is not easy to put a price on. Tripods don't become obsolete, a good one will past many many years.