One of the biggest themes on this site you will see is how saving money and helping the environment go hand in hand. Capitalism, while deemed by many to be harmful to the environment, may actually be it's salvation.
You have to understand that built into the cost of every item is the total cost of the energy used to create the itme. For items that use energy or require energy to operate, you should also factor in all that enery from the product's lifetime into the cost. We call this Total Cost of Ownership (TCO). It's a fundimental buisness calculation that is often neglected in personal buying decisions.
Food Items - Shipping is bad, but a greenhouse can be worse.
Say it's January, and you live in the northeast. You want to add some tomatoes to a favorite recipe. You have a few options:
California grown tomates trucked across the country - $3/lb
Locally grown tomatoes grown in greenhouse - $6/lb
Can of tomatoes - $1/lb
Home canned tomatoes - ~~ free
Which option used more energy to bring to you? Well, believe it or not, the local greenhouse might be the worst option. Heating a greenhouse all winter uses a lot of energy. While trucking the tomatoes all the way accross the country can be very expensive too. With both the "fresh" options, part of what you are paying for is the spoilage of the tomatoes that you don't buy or use. Canning of the tomatoes takes some energy, but reductions in spoilage and the fact they are canned at the peek of the season save energy and money. Of course, the best option here is your own canned variety, but a store canned option would be second best, as the price reflects.