This story is about the Easter Bunny, or in our case, Bunnies.

We have three, and they do the yard work. We play on the lawn, and they mow, aerate and fertilize the lawn, in any weather, without causing any kind of air or noise pollution, other than the output of CO2 from their respiration. But then, their fertilization of the grass stimulates carbon sequestration, too. (Some day I would like to quantify their CO2 balance, and see if they are a net contributor to greenhouse gases, or a sink for those carbon atoms.)
Today I decided to get an idea of the financial costs of owning rabbits vs. owning a lawnmower for lawn care.
Consumption:This year (January through April 2009) I have spent 18.76 euro on rabbit food (alfalfa pellet mix) and 25.00 euro on annual vaccinations.
That is 43.76 euro so far, with an average of 4.69 per month for food. I should expect to spend about 38 euros more this year on rabbit food, for a total of about 57 euros for the whole year. It should be less if I can stock up when the food is on sale, or find a cheaper pet store.
I don't know how much city-treated water they drink, but I think I will call the cost of it negligible, especially compared to the cost of doing one load of laundry or dishes.
A power lawnmower, on the other hand, eats gas or electricity and oil. A liter of "benzine" right now, in our town, costs 1.193 euro. I have no idea how many liters per meter the average lawnmower gets, or how many kwh per square meter. I do know that lawnmower exhaust smells worse than rabbit pee, and doesn't fertilize as well.
Speaking of fertilizer, how much does a lawn need per year in this climate and soil type? This is fairly depleted scraped clay we are on, here. My lawn is the greenest of all the back yard lawns on this side of our street ( I can see them from the upstairs window). So lets assume the cost of fertilizer is too high to be much bothered with purchasing and/or using it, if the neighbors don't.
Start up:A search of the second hand lawnmowers available for sale today on koopjesnet and kapaza, two local online selling forums, revealed a minimum lawnmower startup cost of 20 euro, for a manual lawnmower, used. This is the cost of a new rabbit bought from the local "vogelmarkt" in downtown Antwerp.
One can obtain a bunny for free in several places around here: the local community animal shelter (where tons of pets have been dropped since November last year---including rabbits--and the space is overflowing), local farmers and neighbors who advertise free bunnies from their own rabbits' litters. A seller at the local Saturday market had rabbits for 7.50 (for a baby) to 10 euro (for a 3 month old). Our local pet store sells rabbits for 35 euro. Also, one must have a few rabbit supplies, like a water bottle and shelter. A new water bottle runs from 2 to 20 euro. Rabbit cages and shelters of many many kinds exist and can be bought or built from scraps (as we did at our former house--wish I had a picture!). Heifer Project international has a rabbit burrow system that I like and would like to try one day--it uses about half a bag of concrete (about 1 euro), and you grow a garden on top of the rabbit burrow. A super delux rabbit home with enclosed run, where the rabbits would be eating grass, costs 259 euro at the cheapest warehouse store in Belgium.

The maximum cost of a power lawnmower was in the thousands of euros (the thing was really a truck), but a reasonable used gas mower cost 130, and the same mower was also advertised new for 225. The cheapest used electric mower was 60 euro; the cheapest new electric mower was 140.
In summary: startup bunny: free to 35 euro, plus extras up to 300 euro
startup mower: 20 to 225 euro, plus extras up to 5000 euro

Now it is time to go feed all the pets. Maybe next time I will compare the time costs of rabbit vs mechanical lawnmowers. I might expostulate on dubious side benefits and hidden savings. Then I would like to describe how to make the rabbit mower run and work well for a variety of yard conditions. That sounds fun to me!
I found only one other post about a similar topic so far:
http://geocities.com/theirsisthekingdom/011.htmlI'll keep looking!