Not only the price of fuel, but the relative price of the two fuels. In the US, diesel is a fair bit more expensive than regular gasoline, which cuts down or eliminates any cost savings you get from higher efficiency.
It is cheaper than premium gas in most places though, and given the cars diesel engines are offered in, diesel tends to compete with premium, not regular. The obvious exception are 1/2-1 ton trucks, but in those, diesel engines serve a different purpose than the gasoline ones.
Seems to be true in the higher-end diesels, like the BMWs and Audis and Mercedeses, but the gas versions of cars like the Cruze and the Jetta use regular gas. For people who are cost conscious, I imagine that they'd be looking at the lower-end diesels where the gas equivalents use cheap gas.
Not sure about the Jetta, but the diesel Cruze is so much more efficient than the gasoline version that diesel has to be 1.5x the price of gas for the latter to make sense. EPA cycles tend to underestimate the fuel economy of diesel TDIs and overestimate those of gasoline TDIs. They're pretty accurate for non-turbo gasoline DI engines though.
