The best I can do to answer that question is Australian math for the AWD CX-5. Using the numbers converted from metric this is what you get. The gas engine they sell is a gas 2.0 that gets 34 mpg (the US version is listed at 31 mpg) and a 2.2 diesel that gets 41.3 mpg. The numbers for the 2.0 are 150 hp @ 6000 rpm and 148 lbs @ 4000 rpm and the 2.2 is 173 hp @ 4500 rpm and 310 lbs @ 2000 rpm. So the diesel doesn't rev as high as the gas engine but still revs pretty high. The gas engine needs to be revved up a lot to make it's peak torque while the diesel doesn't. Being an aluminum block diesel you don't have much of a weight penalty. So the diesel should feel like a more powerful engine. The CX-5 has a 15 gallon tank.
So now the math. Assuming that gas is 50 cents a gallon cheaper and is selling for $3.80 a gallon here it'll cost $57 to fill it and you could expect a range of 510 miles. The diesel will cost $64.50 to fill it and go a range of 620 miles. That's an extra 110 miles, that will cost you an extra $12.35 in gas to get that range or a savings of almost $5 per tank. To go 1000 miles in the diesel it'll cost you $104 while the gas engine would cost you $112. If all other engine maintenance costs are the same that would be $800 per 100k miles savings. It would take about 300k miles to offset the cost of the diesel. Doesn't sound worth it does it.
But here's where things get fuzzy. If you test drive the 2.0 gas engine and find it's a dog while the diesel has enough power then you would rule out the 2.0 gas CX-4. The 2.5 gas CX-5 gets 30 mpg (184 hp @ 5700 rpm, 185 lb-ft @ 3250 rpm), that's So lets say you are also looking at the Rav4 with it's 2.5 engine (176 hp @ 6000 rpm, 172 lb.-ft. @ 4100 rpm). It gets 29 mpg in AWD. That's now $131 per 1000 miles. That means the diesel would save you $2700 over 100k miles for the Rav4 and $2270 for the 2.5 CX-5, or about the cost of the diesel engine option. All of a sudden the diesel starts to make sense. If your drive allows you to take full advantage of that extra torque then it should be possible to get an even larger difference in mpg. if you plan on putting lots of mileage on the SUV then you can save money. If you figure in that the used price for a diesel should be about $1000k higher also means it'll help make justifying the diesel worthy. If gas drops to $3 a gallon while diesel only drops to $4 a gallon then you are not going to save.
So the answer is maybe. For some the diesel isn't enough of a savings but for others it could mean $1000 or more. If the extra torque of the diesel is really noticeable and you want it then saving money is secondary.