I don't think a lot of people make a lot of money from it. It depends on what you are selling and the age you sell it at.
I know that even the garden produce I grow for myself and my family and friends just barely nudges out the costs, but I do not grow intensively and I actually have to be careful about quantities or most of it will bolt and be too old to eat. The greatest reward is knowing what went in it to produce it, and knowing it is pesticide free and the huge difference in taste when it is fresh. I really don't like shriveled and soft cucumbers from the store anymore and herbs are so much better fresh than dried. The value of my garden produce has grown since the price of food has skyrocketed mainly because of the price of gas, transportation, and ethanol, not to mention that China is willing to pay top dollar for meat and that means more grains are needed to feed those animals and the grains are more expensive because the animal producers must compete with ethanol for the grain. Now, there is the real rip off. There must be something else ethanol can be made from besides the food grains.
I do grow herbs for the botanical garden plant sales. I grow approximately 20 trays a month and I sell 50%-80% of them. I figured on the material costs are 40% of the sales price(pots, media, fertilizer, plants, seeds, labels, information sheets, promotion). What is not accounted for is water, opportunity cost (although I have assigned opportunity costs to some items to determine price), and labor. I figure in labor, and the time I am putting in weeding, mixing, planting, transplanting, transporting, selling time, etc., that I am probably making way under the minimum wage.
There is also the fact that the garden only has a sale one day a month, so some of the plants will need to be culled to make room for more since I have limited bench space, some of the seeds won't sprout, plants will be unsaleable because of breakage, poor growth habit, pests and disease, and because of timing. One day a month sales are hard to time to get plants at their prime, not too young and not too old. Plants that will not last another month have to be culled to salvage the media and pots for reuse. That is why I limit the vegetables I sell tomatoes, cucumbers, squash and chard. These do not keep so I only have about 10 of each. The one I have the best chance of selling are the Japanese cucumbers, the others are hit and miss. If you sell more often you will have more opportunities to move your inventory.
I try to change things up and try new plants and rotate plants that have been selling for a while but demand has dropped. Sometimes it works, sometimes it doesn't. Sometimes things sell very well for a while, then all of a sudden demand changes and people want something else. To predict what people will want in a given month, you probably need a psychic or a Ouja board. You have to know your market and stay ahead of the trend which means you probably have to have a variety of different things and that takes up space. You will have to decide which is more valuable to have. The opportunity cost, is the loss choosing one thing over another.
Happy gardening in Hawaii. Gardens are where people grow.