The others are more expensive because people will pay it. The other reason is that the other usually add more features, such as dual TDS meters, pressure gauges, double-gasket quick-connect fittings, etc etc. What it comes down to is that the media is roughly the same and the RO membrane is always the same, made in the USA, TFC (thin film composite, for use in chlorinated water municipal supply only), and usually the other are a lower GPD. Bottom line is that you don't need before and after TDS meters, you just need this one
Amazon.com: HM Digital Pocket Size TDS Meter TDS-4 Water Quality Tester: Kitchen & Dining
And you test your output water every so often, when the TDS starts to rise, you change your pre-filters, or you track how much water you make and decide when you're going to change them.
www.bulkreefsupply.com does carry a few better filter cartridges, one is their 3rd stage 0.6 micron filter, versus purewaterclub's 5 micron 3rd stage, but I just plan on replacing that one with the one from BRF when I need to, and will probably order my carbon block 2nd stage from them too. But their system price was 2.5-3x so it just wasn't worth it. Like you said, it works like it should, 0 ppm, and there's no difference between theirs and anyone else's 0 ppm water.
I got the 150 GPD dual DI one and bought a couple shutoff valves & T's at Menards and rigged it so that I can boost my existing household system before the DI when I'm not using it.
