I don't know what you consider a reasonable cost, but expect to pay about $175-$200 for a good system.
1) Be sure the system is assembled using standard parts, and utilizes maintenance components (sediment filters, carbon filters, membranes) that are readily available. Systems made using "quick change" components or custom-made ones can leave you stranded.
2) Be sure there is a hydraulic cutoff switch. It's a mechanical device about the size of an egg that had 4 tubing connections on it, usually behind the membrane housing. When your tank is full of water, the switch shuts off ALL water flow; without it, the flush water would keep flowing. It's rare that the system doesn't have one, but I have seen them, especially in systems folks expect to operate manually.
3) If your water is chlorinated, you want a 5-stage system, as the second carbon cannister helps protect the membrane for longer life.
4) While not absolutely critical, I highly recommend that the system be assembled with quick-connect tube fittings, as it makes maintenance a lot easier.
5) Think about your water volume needs, frequency of watering, and storage. RO systems are not "on-demand" water supplies. 100 gallons per day (gpd) is only about a cup a minute, and it would be really hard to water anything well at that rate. Standard systems come with a 3.2 gallon bladder tank, having a usable capacity of 2.25-2.5 gallons. If that's not enough to water once, you'll need to store water in other containers (which, if you do that, may negate the need for the bladder tank in the first place.
6) Think about the flush water stream. A standard system is sized to pass 3 gallons of flush water past the membrane for each gallon of pure water produced. If that pure water stream is going into a pressurized bladder tank, as the back pressure increases that ratio increases, giving an average for the entire refill of something in the 4:1-5:1 range. They are specified as "4:1". Higher efficiency systems are available; mine, for example, is 2:1. If water conservation is important to you, or you pay a swerage fee, a higher-efficiency system is worth considering, and should not cost any more.
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