You can save money on the RO by harvesting rainwater. And the savings are ongoing since you won't have any extra water bills to pay.
Depending on the cleanliness of the roof you collect from, the pipework and the containers you use to store the water, you may even be able to use it raw without running it through an DI unit.
As you'll only be using small amounts of water to begin with, it will save you the capital cost up front. When you start earning later, you can then invest in a RO unit from revenue.