I have seen a lot of garden blogs, most of which track their harvests or costs in some way. Some take the time to weigh each type of veggie individually, some show total weight, some show initial costs, some show price per pound.
Paying attention to these calculations can help a new gardener to determine what pays off and what doesn't; there are a lot of variables to consider though, and several different methods of calculating.
To be as accurate as possible you should weigh your produce individually (I am too lazy for this), and find several prices for them at the places you would normally buy them. At the first of the strawberry season a pound of berries could run about $3 at my local grocery, in mid-season I can pick them myself for $1.40/lb, purchased organically the price goes higher, out of season the price is higher, and so on. For strawberries I would calculate based on the grocery store price, because my own strawberries are fresh, organic, and i get them throughout the season and have some for freezing. Therefore, I get berries out of season, organic, and picked at their peak. So $3/lb. is not unreasonable at all. Considering the only costs for the strawberry bed have been the $5 starts, and a $8 bag of slug bait every year, I think they have paid off well. Even if I just went by this year, my costs even out to $1.30/lb. and this was not the best year for strawberries. Last year my harvests were much better and i averaged around $0.70/lb.
Some things pay off quickly, others don't. This is why I prefer not to be so accurate, my broccoli never really pays off, but the herbs pay off more every year. The system i use is to keep track of my expenses (easy enough if you use a money tracking system that includes a Home & Garden category), then keep track of my total harvest weight. If you divide expenses by weight you will get cost per pound, with every pound of produce harvested your cost per pound will go down. A good range to aim for would be between $5 and $0.50 per pound.
My calculations are going to be off a bit this year since I put quite a bit of money into the grow room, and bark for the entire yard. On the other hand, this means next year's expenses should be very low, I have tons of seed left and the landscape part of the garden is done, and the grow room is already there.
The main calculation systems i have seen are:
Income: Total Costs - (Price per Pound*Pounds Harvested) = Balance -$500 -($4*65) = $-240 With this system you can determine at what point you reach a zero balance, and when you start to earn more food than what your expenses equal. Potentially you can show your spouse a spreadsheet with a $200 or $300 balance at the end of the year and explain that you saved that much money in groceries.
Price Per Pound: Total Costs/Pounds Harvested $500/65 = $7.69/lb. PPP shows how much you paid per pound of food. It doesn't require that you know the average cost per pound of the food you grow, which the Income method does require. It also averages out all the plants, and all the expenses, so cataloging the individual food weights isn't necessary.
Detailed: Cost for Vegetable Plant - (Price per Pound*Pounds Harvested) = Balance for Vegetable
Broccoli (-$0.99) - ($2*5) = $9.01 (Repeat for each vegetable or fruit). This is a great way to really see which plants are paying off. The downside is that it is much more work to weigh each vegetable separately, then divide up your seed packet cost by the number of seeds, or your fertilizer cost by how much each plant type used, etc. Still it can be helpful in determining if it's really worth the time to keep planting shelling peas when frozen organic shelled peas are so inexpensive. It can really help a gardener working with a small space get the most value from their garden by planting high value plants like perennial herbs, berries, sugar snap and snow peas, or squash.
Each of these systems has a purpose, and each has pros and cons. But calculations like these are an important part of taking garden notes so you can continue to improve your garden's production year after year.
One last calculation that may be particularly important to small space gardeners like myself is determining your pounds per square foot. VegPounds Harvested/Square Feet Used = Pounds Per Sq. Ft. In this case if I planted 4 sq. ft. of broccoli and got 3 pounds of harvest from it my answer would be 0.75 lb./sq. ft. Meanwhile two square feet of zucchini gave me 10 lbs of zukes. The answer there would be 5 lb./sq. ft. So if I really like zucchini it is obviously much more useful to plant a zuke than broccoli (Since I don't like zucchini though, I'll stick to planting broccoli instead).
Anyway, I hope this will be helpful in keeping track of harvests, please feel free to share if you have a system I didn't show here.