I recently described the first set of vegetables that I started from seed this year. There are a number of reasons that I choose to start my plants from seed. One of the most important reasons is that it is a lot cheaper.
To illustrate this, I will estimate how much it cost me to start a 72 cell tray of onions and a 72 cell tray of lettuce this month. I will not include the costs of things that can be used indefinitely (such as light fixtures, and tables). I will try to prorate the cost of things that can be used multiple times (like light bulbs). At the end of the article is a justification for the costs shown in the list; generally I try to overestimate the costs.
Seed starting mix: $2
Seed trays: $2
Light bulbs: $1
Heat mat: $1
That is the total for the the onion tray and the lettuce tray. If you divide that by 2, the total for the lettuce tray by itself would be $8.05.
$8.05 for 72 lettuce plants is roughly $0.11 per plant. If you buy lettuce plants at a local hardware store or nursery, you can probably expect to pay at least $1 for 4 plants, or $0.25 per plant. Starting my own lettuce seedlings is more than 2 times cheaper than buying plants! The savings are even greater for plants that you direct sow (like squash), because the only cost is the seed and water.
In addition to the savings, starting my seeds gives me greater control over them. I know that they were raised organically, and I also know that they were well cared for and not neglected. There is also a sense of satisfaction knowing that you grew your plants from seed.
Seeds: I used less than half of a package of both onions and lettuce, and each packet was $2.99.
Seed starting mix: I use a compressed seed starting mix that is $2.50 for 6 quarts (rehydrated). I only used 3/4 of it for both trays.
Seed trays: I buy these at the local garden supply store for less than $1 each.
Water: At my water rates, $0.10 should buy me about 15 gallons of water, I doubt that the trays will use more than that before they go outside.
Light bulbs: The two trays together will have 4 - 4 foot fluorescent bulbs, which are roughly $8 for a 2 pack. That is $16 for the 4 bulbs. Given that the bulbs are expected to last 30,000 hours, I should be able to count on them lasting for at least a few years and for multiple trays a year. I think $1 is a generous estimate.
Heat mat: This heat mat was $20, and has already been used to start over 10 trays, and will probably start that many for years to come. At that rate, if it lasts 4 years, it will be 0.50 per tray.
Electricity: The 4 fluorescent bulbs draw about 32 watts each. If they are left on for 16 hours a day for a month, they will draw 4*32watts*16hours/day*31days = 63.5 kWh. At a rate of 10 cents per kWh, this would be $6.35 for a month. I rounded to $7 to account for the low cost to run the heat mat for a few days.