If you're feeling really energetic, you could go to the county recorder's office, and look for deeds transferring the properties from the current owner to new owners, and then calculate the sales price by dividing the documentary transfer tax by .00055. The result isn't perfect, but will be within $1000 of the actual contract price. The Santa Clara County Recorder's office will let you look at the index of recorded documents online, but you must go to their office (or pay a third party) if you want online access to the document images.
If this is a one-time event for you, it's probably easier to find a real estate professional with MLS access who can look this up for you without spamming you to death in the following weeks/months.
If this is data you need on an ongoing basis, there are several vendors - one to look at would be Realquest.