Don't get a cheap one. Proper wine hydrometers come with a certificate to show they have been tested. I recently bought a professional wine hydrometer which is calibrated in beaume, a measure which shows potential alcohol ie 10 degrees beaume = 10% abv if fermented to zero.It cost $38 aussie.
If you don't plan on getting into winemaking in a major way a cheap one is fine because it is the relative readings which matter. the change in the readings shows the progress in the fermentation. You measure before and during fermentation which shows how fast the wine is fermenting, some recipes require you to rack before primary is finished which is around 0 beaume or 1.000 SG.
Most on this forum use Specific Gravity (SG) instead of beaume, a reading of 10 beaume = app 1.074. it doesn't matter too much which you use, its the change that matters.