I have built homes for personal use, and bought and sold land, and as a realtor, have helped clients buy land. There can be a lower overall cost on some properties for buying the land and then building, but first you start with soils engineering, the building restrictions, tree mapping and measuring, surveying. That is just to see if you can really build on the land. There are a lot of costs even before you have stuck a shovel in the ground! If the site is steep (which most in the bay area are, now) your foundation costs will be huge with how deeply you have to sink the piers. Then there is the water meter, connection to the sewer, etc. A deal? to live the dream of bulding your own place, maybe, and the whole thing will probably take at least two years.
there are some good deals on land outside of Livermore, but the good deals are the properties where you will never be allowed to build - you may bring in a travel trailer and spend your weekends there, fishing and getting away from it all. Good luck!