You can find numerous seed trading sites on the internet as well as in some gardening magazines. You could receive hundreds of seeds for no more than the cost of a self address stamped envelope or a share in some seeds from your own garden. I like to save my seeds in spice jars, with the shaker tops with large holes. The makes seeding in the spring easy and quick.
You can make your own seed tapes or mats with newspapers and a homemade paste of flour, water and a water-soluble fertilizer the consistency of thick gravy. You can also make your own seed starting containers from newspaper, paper towel and toilet paper tolls, cardboard egg cartons, paper cups, etc.
Trading plants with other gardeners is a great way to get free plants, as well as meeting new gardening friends. Consider having a plant trading party in the spring and invite your friends, and let them invite their friends and so on. Before you know it you may have gardens full of plants that cost you next to nothing and lots of new friends. If you purchase plants at a nursery look for plants that are full and may even have more than one plant in the pot.
You can separate and divide larger plants. Look for plant sales at schools, churches and local garden clubs. They are reasonably priced and the selection is right for your area. You can also get free bulbs from florists who use only the cut flowers and discard the bulbs.
Composting can be done at the cost of only a spot in you yard. You can compost only anything from coffee grounds with filters, fall leaves, grass clippings, shredded newspapers, sawdust, wood ashes and vegetable or fruit kitchen scraps. It will add much needed organic material, improving your soil consistency while reducing the cost of fertilizing.
Make bug spray and plant boosters. A mix of ½ cup hot peppers pureed with 2 cups of water strained and used in a spray bottle will keep insects (as well as cats and dogs) away from your gardens. You can find as many different recipes as there are problems on the internet.
Be a curbside collector. I got a perfectly good Adirondack chair for free (after re-attaching the arm and adding a new coat of paint) that some one threw away and it was as good as new. Yard sale items can give your budget a boost, too. Buy in the off season and end-of-season clearance sales. Recycling may give your garden a lift for free. I used a set of glass light globes to a ceiling fan to add candlelight to the edge of my garden, at the cost of only the votive candles. You are only limited by your own imagination. Happy gardening!