What is the source of you tap water? And what is the TDS? With this hobby, I have found it best never to say that you can’t do something. You will always find someone who then tells of the success story of doing that “something” for many many years.
That being said, my experience tells me that if you like hair algae, diatoms and an every-increasing TDS from evaporative make-up water, and, not to mention the plague and pestilence, than tap water is the way to go. If it were me, and the cash was that low, I’d be buying my water at the LFS until I was able to afford the RO/DI unit.