The Sparkfun breakout board uses 2.8V supply to the chip, its internal reference is rated as 1.35V, so I believe it will be outputing 1.35V at zero rates, which is roughly a count of 276 on a 5V Arduino, so those ADC values make sense.
You should see them vary while the unit is turning, either increasing of falling depending on the sense of the rotation in the relevant axis - its not easy to turn something at 500 deg/s by hand unless its on a longish set of leads(!).