Just as an aside, compare the cargo area of the Mazda 2 to a Honda Fit. The Fit is much larger and has a flat floor. A Fit Sport automatic is actually cheaper and has seventeen horsepower more. Going to be tough competition.
Diesels, ahh, diesels. Their popularity has peaked in Europe. In a small car, an extra $1500 for diesel is going to be a long pay back and any potential out of warranty engine issue on a diesel is going to be extremely costly.
Speaking from experience, people buy diesels in trucks because they are cool, not for any performance/economy benefit. Recent emission systems that use fuel to burn off soot have really increased fuel consumption. If you want to see what costs the least to run, look at fleets. I am an engineer and I work on remote construction projects all the time. Practically all the pick-ups I see are Ford F-250s with gasoline engines. Kewitt has thousands of them, for example, nary a diesel to be seen. Problem is, according to the mechanics I talk to, is that the diesels are extremely sensitive to fuel quality and most repairs cannot be made on site, meaning the truck has to be sent to a dealer, which costs a fortune. For this reason they stick to the Triton 5.4 which is as reliable as a frying pan!