Buildable lots in this area can be extremely pricey! As was stated, the most desirable lots are downslope because they are easier to build on and as such they are rare. I would also say that the dollars per square foot are closer to $350 and even higher to get the high end finishes, and green amenities that buyers are looking for.
The current market price for a lot in Claremont and Berkeley Hills ranges from $50-$90 a square foot. The price depends on the quality of the view and the 'buildability' of the land. Flatter and downslope lots are more desirable than upslope lots and usually less expensive to build on.
In building a home, the rule of thumb is $300.00 per sf, but you need to add on the cost of land, permits, etc...