after reading 100s of web reviews here is my analysis of cost per year for robotic units:
cost per year = cost of unit/years of warranty
my reasoning is that the motors on these units fail at a very high rate and once the unit is out of warranty the cost to repair is too high. I'd guess the mean time between failure for a pool robot is about one year. So once a pool robot is out of warranty, it will probably fail within a year. add in the cost of shipping to repair centers and I think the formula above is roughly correct.
Here's an example of how the formula can help guide purchases. I'm considering the dolphin dx5 with a two year warranty for $750 vs. the dolphin supreme m4 for $1200 with a three year warranty. So the dx5 cost/year is $750/2 = $375 while the m4 is $1200/3=$400. The m4 for $25/year seems well worth the extra spend. also, since the cost is tied to warranty period, it's very important to make sure your credit card will provide a year of extended warranty at no cost.
another point worth making is that robotic manufacturers have access to real data whereas my data is just anecdotal. I wonder if they have any incentive to improve motor longevity. The best financial outcome for the manufacturer is a robot failing the day after the warranty ends.