farmingvillefarmer,

I was doing a little math while potting up vegetable starts today.

A 2 cubic foot bag of soiless potting mix is about $10.00.
There are about 240 cups to a 2 cubic foot bag. Each cup cost about 4 cents. I think a 4" pot holds about 2 cups.

4" pot = 8 cents plus your seed. We sell our 4" pots at $2.00 each and a flat of 18 pots for $30.00.

The only plants we pot up to a 1 gallon nursery are tomatoes and peppers. I believe they were $5.00 last season. I like to sell the plants before they get to this size. Less material and labor.

We sell at a premium price, for if they don't sell, we plant out. We sell produce also.

I'd also recommend that you grow cherry tomatoes. The market last year, here, seemed to have a shortage and sold out quickly.

We haven't grown herbs, maybe this year. Herbs take a little more to get started and grow out. You should charge more. Hopefully someone else can help you with them.

Eric

Eric is pretty spot on, the smaller sizes cost you less in the long run but the larger sizes are also good to have on hand.
Here are some more measurements.
http://americanclayworks.com/catalog2008/mixes.pdf
page 110 has the good stuff. Remember, these measurements are for a compressed bale, they fluff up quite a bit.

As for sales, make sure your flowers are blooming.

also this link seems useful also, ITML is the most standard.
http://www.americanclayworks.net/acw_weights_and_measures.html

I assume most of these measurements are available on other sites, but this is the place I am familiar with.