Answer: Since the site is sunny, wooly thyme is a good choice, Sagina subulata (Scotch or Irish Moss), or Blue Star Creeper. The foliage will contrast well with stepping stones and soften the edges of the stones, and all take some light foot traffic. If the site is shady to partly sunny, Soleirolia (Baby's Tears) will produces a carpet of mossy-looking green with its tiny cup-shaped leaves.
I can't quote prices - retailers set their own. If you're looking for the most economical method, purchase your groundcover in flats rather than individual pots. You'll get lots of plants at a reasonable cost. Groundcovers such as these can be cut into 2" or 3" squares and planted. They will fill in very quickly if planted on amended soil and watered regularly. Best wishes with your project!
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