A cheap power supply isn't a good place to try to save money, although it's often the first place less experienced builders will try to go cheap. As a general rule, you want a well-built power supply that conforms to modern technical standards, and you can expect to spend at least $40 on it. Decent PSUs priced under $30 simply do not exist.
Just a quote on building an entry level gaming pc. the 670 isnt entry level, so the gap is even larger