Walmart offers the Danskin Now Women's Essential Active V-Neck Shirt in seven colors (Cabo Blue pictured) for $4. Opt for in-store pickup to dodge the $5.99 shipping charge. That's $2 off list and the lowest price we could find. It's available in sizes XS to M, although not in all color/size combinations.
Nike offers its Nike Women's Dry Miler Running Tank Top in Team Royal/Team White for $19.97. Plus, Nike+ members receive free shipping. (Not a member? It's free to join.) That's the lowest total price we could find by $3. They're available in sizes L or XL only.