You have to keep in mind that the assessor's value is not the same as market value, and is used mostly as a gauge for assessing county taxes.
What you can take away from it, however, is relational value. The downward trend you see is a direct reflection of the recent housing slump we've experienced and is the same across the board here in Tucson/Sahuarita. So yes, it's probably normal.
How does the home look when looking at other comparable properties? If you have a Realtor they should be able to provide all this information for you quickly and easily, and perhaps provide more insight into the reasoning behind the recent price trends. Rancho Sahuarita is a great area and there are definitely a lot of deals to be had, even from new builds.