How often do they need replacing and how much usually
Looks like the DI resin cartridges are about $20 each. I couldn't find the specifications for the resin capacity, which would be needed to calculate its capacity for removal of ionic components that make it through the RO membrane. It's also not clear whether you can actually refill the DI resin cartridge with loose resin, or you have to toss the whole assembly, plastic housing and all.
As far as cost, it appears that
Amazon will sell you a
3-cartridge replacement kit (DI resin, sediment and carbon) for about half of the cost of the unit.
When considering a RODI unit, there's a few things in play that may affect the lifetime of the unit and the quality of the water it produces. Specifically, it will depend on the incoming water's characteristics and whether or not the user is on well water (no chlorine/chloramine) or on municipal water (typically sanitized with chloramine). In the eastern US, municipal sources that use surface waters, such as lakes or rivers, will tend to have tap water that's fairly low in dissolved ionic solids (TDS <200 ppm). In the western US, some hobbyists sarcastically call the municipal water "liquid rock", since it has TDS that is often 700 ppm or above. Ultra hard municipal water is more difficult to successfully purify to RODI standards, and it typically requires more complex and expensive systems to run in an economic manner, such as using two RO membranes in series with a booster pump. If you're on municipal water, you can look up the water authority's test results on line and get an idea of what the typical incoming water will be.
As for purchasing a system, the RO buddie is probably ideal for freshwater hobbyists that don't require ultimate purification the way saltwater hobbyists do, for people with smaller tanks, and for folks that live in the eastern US where water costs are often dramatically lower than in the western US so that an RO system's waste water isn't as much of a concern.
You can, by the way, always purchase a traditional, modular system from Bulk Reef Supply like their "value" systems (about $150), and add on to them if you determine that you need extra carbon blocks to filter chloramine, or you want more water production for bigger tanks. I believe they sell sink adapter fittings to hook their systems up temporarily as well.
