Therefore the iPhone share of total search ad spend is about $3B. This link puts the overall market (inclusive of Android) at roughly $9B in 2014. That makes sense if you assume iOS to represent roughly 1/3 of devices.
Isn't the $9B for all mobile advertising, not just mobile search advertising? According to a Glodman Sachs report for 2014, back then they estimated that iOS advertising accounted for 75% of Google's revenue on mobile. That would make the Android search ad space worth about $1B.
It's not available online. The story originally came from the NY Times, but I think Business Insider has the best info as they directly quoted a section from it.