I think Alan has a point...if you reduce the tractor hrs by 60 and usually the efficiency of the ZTR is higher so maybe 40-45 hrs on a ZTR vs the tractor annually. But the ROI?
On average I would suspect a 25-30 HP is around $18-20K with a FEL and MMM. The MMM probably worth $2500. A ZTR typical is $6-10K for the commercial ones, so deduct the MMM cost and increased efficiency and less hrs on the tractor may give it a longer life and better resale value.
Of course it you put 60 mowing hrs on a tractor for 20 yrs thats 1200 hrs.. and 45 hrs on a ZTR for 20 yrs thats 900 hrs.. If you own the tractor already then you have added investment say $7500 for the ZTR over 20 yrs thats $375 a year.
Feeding and maintenance probably would be a wash, fewer hrs on each piece, and diesel vs gas in a ZTR with fewer operation hrs may be a wash also.
I guess it comes down to resale or end value in 20 yrs.. a $18K new tractor with 2000 hrs vs 800 hrs after 20 yrs.. then add $7500 to the equation for the ZTR. I dont think the ROI is there for this.. just my opinion.
This said I have wanted a ZTR for a long time and last year got a Ferris as i increased mowing area by 1.5 acres and the
B21 TLB never will be a mower..and the 21 year Cub Cadet still ran OK it is EOL without some engine and other work in the near future..will keep the Cub but it more of a utility mower/cart puller now..
Then there is business investment many have here with capital equipment purchases..the ROI for the ZTR could be very compelling.
It really comes down to toys I think and while the ZTR is more time efficient, better cut etc, the bottom line they are really fun to run...and a change of pace from the tractor, no matter the color.
I grin when I mow now.