Here's my two cents about saving money making water....
1. I'd suggest a "full sized" RO/DI unit. The RO Buddy works but requires you to buy their replacement filters....which are small and therefore require sooner replacement. With full sized filters you can shop for the best bang for the buck on filter changes....also for a high rejection membrane....you ideally want at least 98% rejection.
2. Install a three way valve between your RO unit and your DI canister. The reason is that on start up you get breakthrough TDS from the RO unit that otherwise would be removed by your DI causing it to deplete much sooner. What you do on start up is flip the valve to run the water to waste for about five minutes and then flip the valve to run through you DI canister. My breakthrough TDS spikes to 50 to 60, but after about five minutes it's down to 2.
I get about a year out of my DI resin this way.