Road noise always has a detrimental effect on house values. How much it bothers you depends on how sensitive you are to it. In practice, after you have lived there a few months you may not notice it but you absolutely have to consider the fact that you will want to sell it one day and the road noise will probably be worse then as traffic in San Ramon is increasing.
It may seem reasonably priced and If you have an agent (which you should, to protect your interests), he or she should be able to show you if it really is good value for money. If the road noise is significant, the effect on true value will also be significant.
If you want an assessment of the value, let me know and I will be happy to help.
Bernard Gibbons, Realtor, e-PRO Certified Internet Specialist
J. Rockcliff Realtors, 15 Railroad Avenue, Danville, CA 94526
Phone (925) 997-1585