The one you posted the link to and the Aquasafe are of comparable quality and are a decent enough value for the price but are much more expensive to run in the long term.
There is a good reason that other units cost 3 times as much. The Aquasafe unit is about 90% efficient, the ebay one you posted claims about 92.5% Depending on your source water this may be good enough or maybe not, take this for example: Source water is 200ppm on a tds meter, Aquasafe membrane does about 90% removal so your "RO" water still has a tds of 20ppm. This needs to be removed by the DI resin and it will eat it up quickly. Take that same 200ppm source water and put it through something with a Dow Filmtec 75gpd or similar membrane that can do a 98% removal and you only have 4ppm for the DI resin to handle. The resin will last 5 times as long, some people only get 150g out of their DI cartridges in cheaper systems.
This more effective RO membrane is the biggest reason a good unit costs more but other factors come in to play as well. The sediment filter is only a 5 micron, this allows a lot more stuff to pass through than a .5 micron sediment filter would and so the carbon filter gets plugged faster leading to chlorine break through. Also, the carbon prefilters that come with an Aquasafe unit are 5 micron and good for about 5000 gallons at best. Keep in mind though that all of the water that goes through the unit goes through the carbon filter so if you are getting a standard product to waste ratio of 1:4 you need to remember to switch your carbon filter after 1000g of usable water. A good .5 micron carbon block like the ones that come with better units does cost a little over twice as much but it will last 4 times longer. It will also do a much better job at chloramine removal if your water supply is treated with it. This means that the RO membrane will better protected and last longer. The DI cartridge won't have to clean up the ammonia left over from the chloramine either and so it will last longer as well.
Just as an example, I put about 100g a week through my Spectrapure system to get the 20g of product water I need, fairly low usage. I switch my sediment and carbon filters every year or so when I notice chlorine showing up and the DI about every 9 months. I'm just about up to 3 years on the same membrane. That's with 250ppm source water.
I don't know how hard the water is in Lethbridge but if it's anything like Edmonton or Calgary you're going to be swapping filters like mad.
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