To save money on groundcover, plan on planting it in late summer or early fall. Late in the season, you should be able to find discounted plants, as garden centers are looking to unload stock before winter. Look for plants that are ready to burst out of their pots—these should be easy to find, as they've been growing all summer. Then, at planting time, divide each plant into two or three smaller individuals. Plant them separately, following the recommended spacing on the plant tag. In the end you've bought, say, 3 plants, but planted 6 to 9. The divisions will settle in easily during the fall, developing a good root system, and take off running in the spring.
Image: Chocolate Chip bugleweed (Ajuga reptans 'Valfredda'). Courtesy of Walters Gardens, Inc.