Came to say exactly this.. There's one on there right now that's right at 25$ that i'd been looking at this week:

https://www.amazon.com/Ade-Advanced-Optics-Salinity-Refractometer/dp/B005ES6MOQ/ref=sr_1_10?ie=UTF8&qid=1522523470&sr=8-10&keywords=refractometer