The only way for accurate info would be from notary records showing the prices property exchanges for. I dont think they issue any! Also due to the b money these arent exactly correct either!
Wherever it is you are buying, if you find a decent honest agent (yes I know!!!) then they should be able to guide you to what proeprties actually trade at.
Im not going to knock the valuers as they have a hard job, but if they ran on the lines of the UK and used comparables, it would be much more accurate for buyers and sellers.
Depending on the unit, they are anything from 15 to 40% out.