No matter how much you garden, whether it's one pot on a windowsill or a full fledged garden, there's no sense in spending more money than you have to, to do it.
One good way to save money is to save seeds from year to year. A lot of seeeds on the market are hybrids which means they won't always produce true, so if you haven't already, start growing heirloom vegetables, which produce true to their seeds.
The easiest seeds to save are things like beans and peas. Just let a few pods dry on the stalk, then pick and shell them and save them for next year. There are many sites that tell you how to save seeds from specific vegetables, so I won't go into that here, but do save them when you can.
To add to your savings, trade your excess or especially good seed for other seed you might need. Neighbors, family and friends who garden will all benefit as much as you do.
Keep saved seeds dry and not too hot and away from light. I keep mine in an old shoebox, in envelopes saved from bills that I pay online. Junk envelopes work, too. Label, date and seal and that's it.
Just so you will know, there are whispers that seed saving and sharing will become illegal as corporations continue to try to own our food. For your own sake, keep up on the news there and make your choice. I personally will go down to the bitter end, protecting our freedom to save and share seed as freely as we should.