I used a plastic hydrometer for more than 10 years, it was dead on any time I compared it to a refractomer. Then one day, nope, it was way off. They are not reliable, and a refractomer is a more accurate instrument. Or, you could use a lab grade glass hydrometer, these are very accurate.
And paying over $100 for a $50 instrument? Hmm, a foolish Steve and his money...you know the saying
I'm not even sure you can buy one for that much?