American Apparel has come out with 2 amazing dresses that let you tie the top in a variety of ways to create the look of your choice as a halter, 1-shoulder or strapless ... you can even roll it down & wear it as a jumper or as a skirt with a self tie belt! And the best part is that they're both under $40!
The Cotton Spandex Jersey Bandeau Dress comes in 19 colors & costs $36:

And the Cotton Spandex Jersey Bandeau Pencil Dress comes in 20 colors & 2 polka-dot prints, for $34:

[Note: Tops from both dresses can be worn as shown in all pictures]