A $3 packet of seeds can yield 5 pounds of lettuce, 8 pounds of green beans, 20 pounds of carrots, or 120 summer squash. Zucchini will indeed give you great veg for the buck. They are the hyperfertile rabbits of the vegetable world.
Depending on the variety and the care it receives, one tomato plant can yield more than 10 pounds of fruit, and in a packet of seeds, there are some 30 potential little tomato plants. That's 300 pounds of tomatoes per pack.
That's a return on your investment. Heck, after buying one packet of cilantro seeds a couple years ago, I let some plants go to seed and have had a self-sustaining cilantro pot ever since. Multiple plantings during the growing season reap multiple harvests.