Purchase younger plants (or plant your own seeds) The younger the plant the less expensive.
When planting young perennials remember that they need plenty of space when mature, general rule of thumb, they will double in size the first year.
If budget is your main concern, use Perennials as they come back every year.
You can propagate more plants from the perennials you currently have growing, or swap with a friend.
Consider holding a Plant- Swap Party, in which you invite several friends to come, each bearing a seedling or a few bulbs to share
Consider planting Ornamental grasses along the back edge of your flower gardens to make them really stand out. Attractive ornamental grasses include Pennisetum Fountain Grasses, Japanese Blood Grass, Blue Fescue, Maiden Grass, Blue Lymegrass, and Prairie Dropseed.