It will be interesting to read the responses but a couple of quick questions.
1) Why aren't you figuring in fixed costs (equipment, land, etc) as they are part of the costs of gardening.
I could see figuring in the cost of the land if you purchased the land just for gardening. How to separate land cost for living, running livestock, etc. from the actual purchase price of your garden plot? Not really feasible or relevant for this kind of project. I have to pay to live here anyway, why not plow up the yard and sink a few seeds to defray the cost?
The TroyBilt that my dad purchased 30 years ago has probably paid for itself and any and all repairs many years ago. The savings in food costs the very first year I ever gardened paid for the whole cost of a new tiller. Every year after that, I garden for the price of fuel...not much.
2) Are you going to figure in a labor charge? If labor is free, I'll be posting where and when to show up for work here. Thanks in advance for the free labor.
I'd love to see how much time each person spends in their garden working in it. Drinking coffee on a Saturday morning admiring your work counts as break time or could figure in the value of not having to go somewhere to pay admittance to a nature preserve or aboreatum (my spelling is horrible).
As the physical labor increases my cardiac output, thereby improving my circulation, lowering my blood pressure, increases endorphins that consequently elevate my mood and helps me cope with every day stressors.....let me see.....how does one calculate in the savings on better physical and mental health, not to mention better interpersonal relationships with one's family, neighbors and co-workers? Less trips to the doctor, less money spent on meds, less time spent with a therapist and/or family counselor, better job performance, no money spent on gym memberships......gee, I think the savings are enormous compared to a little labor charge, don't you? 
Now....to calculate the savings on health costs defrayed by eating homegrown, organic veggies and fruits...hmmmmmmm....... :bee
If I just use seed, plants, fertilizer costs, my garden is a financial gold mine. I run a 60' X 40' garden and a 30' X 30' sweet corn patch. My sweet corn patch is very unreliable because of wild life. Some years they get all of it--or seems that way. I guess I could shoot them and try and convince my wife that eating raccoon, skunk, opossums, etc are good eats. God forbid that I should shoot bambi when they are in sweet corn patch.
So if I convince my family that eating raccoon is good, what is the value of a lb. of raccoon?
