Full or part Sun? Full or part Shade? Are Perennials or annuals in the bed? You might want to consider native perennials. As far as low cost, it will be later in the season but places like Lowe's discounts plants that are starting to look bad. They usually create a clearance area. Pretty easy to bring them back to life. The other option is Craigslist. Sometimes people offer plants at low to no cost.