Apart from zonal values, you can always have the property valued by a property appraiser. They give you an idea of the market value of the property taking into account is size, location, improvements, value of adjacent property which was recently sold. Try to check your yellow pages. I tried one before.

Medyo merong damage pero at least they'll do some of the due diligence work for you like verifying the validity of the title, etc. I spent arount P2.5k but that was years ago pa for a 2-storey house which I was thinking of buying. At least, if you're the seller, you'd have a more accurate gauge of the value of the property you are selling.

Good luck!