I understand what you're saying, but I've probably only used about 100 gallons of diesel in my big bota this past year I am guessing I will get back about $0.80/gal. or about $80 overall. For my ZTR I used at most maybe 30 gallons (Never mowed in July b/c of heat) On top of that, a dedicated mower like this will probably get used for about 45 hours or less each year. (30 weeks * 1.5hrs/mowing) I put a total of 15 hours on mine and used it all of last year and had 1.5 acres to mow (takes about 30mins). I will have 3 acres this summer after some additional clearing.
In the end...what I am saying is if you have a cut and the ztr will be just for mowing, you may not use the ztr enough on your own property to make the diesel ztr pay for itself. On top of that if you say a gasser is good for at least 1500hrs, you'd be able to use if for 30 years at 50hrs/yr. So, for a cut I would say diesel is the way to go, but gas for a ztr unless using it commercially.