You can get a standard probably at a good LFS that will be a liquid solution you drop on it and it has a specific...specific gravity if that makes sense, lol. Basically the contents of the bottle should always measure to the same amount.
I would bet that the more expensive one would last longer or need less adjusting. I have the Portable Refractometer and got it from Foster and Smith.(I'm not sure if it ever goes off sale.) I have had it for a year and it is working great so far. The only thing about it is it has a large scale. "Dual scale, 0-100 ppt salinity and 1.000-1.070 specific gravity" So basically the resolution on it is smaller. If you want to measure for your aquarium only and want to get a real specific level, or if you want to keep a real close eye on what is going on with the tank, you might want to consider something with a smaller scale. If your eye sight is horrible and you don't have contacts the smaller scaled refractometers might be a good idea.
Last summer I mixed hundreds of gallons of water in a lab and measured specific gravity out in the field in a million different places. I used a couple different refractometers and used them…. a couple hundred times, lol.
Jon