I have that $30.00 Ebay refractometer, and I use the MoreBeer Excel spreadsheet to calculate the pre-boil, post boil, and final gravity reading.
It's been spot on so far, but I do miss drinking the hydro samples...
Just for what it's worth, the spead sheet is required once the yeast starts to create alcohol in the fermenter. You have to have a begining Brix reading to use the spread sheet. Once you start a spread sheet for any given batch, then you just plug in the new Brix reading, and the spread sheet calculates the gravity.
The Morebeer web site has more info, and Bobby_M (OverBrook Brewery) has a better You Tube video that includes the Morebeer part.
The refractometer is a great tool for better brewing. Get one that has the ATC (automatic temperature calibration).