I've been using the same £4 floating hydrometer for twenty years. Works perfectly well for brackish aquaria. No brackish water fish needs accurately made seawater, so the 'tolerance' of a floating hydrometer is absolutely fine. You aren't going to harm scats or monos by keeping them at SG 1.015 instead of SG 1.014!
Research-grade refractometers cost $1000-$6000. Anything for $50 is going to be massively compromised and certainly nowhere near as accurate as some fishkeepers think. Just because a thing *says* SG 1.012345 doesn't actually mean it can measure to that degree of accuracy.
There is a
hysterical thread at Reef Central (not sure I can post the link) about refractometers versus hydrometers.
My marine biology lecturer at university estimated salinity by taste. The aquarist running the Shedd Aquarium in Chicago uses a swing-arm hydrometer. What matters more than the tool is technique. A hydrometer calibrated for the temperature of water being tested will, when used properly, outperform a refractometer used sloppily.
Cheers, Neale