No, 100% properties are not underdeclared. I bought and sold a few times before I became resident and always insisted the full price. If the value is much lower the you are faced with a huge capital gains bill. A decent lawyer would advise a non-resident to have the full purchase price on the escritura.
Its different if you are resident as its very rare to pay CG tax. The tax authority are clamping down on this practice and I know people who have been fined for doing this. Its harder to do if the property has been built during the last ten years as they now have a chart of values.