These are some I would've worn to pref ranked in order from lowest price to highest. I tried to find a range of prices and looks (safe to daring and very dressy to sort of dressy).
Colorblock Satin Dress, Forever21, $22.80
Pleats to Meet You Dress, Forever21, $24.80
Poised Sheen Shoulder Dress, $29.80
Pleated Rosette Dress, Forever21, $32.80
Rosette One Shoulder Dress, Forever21, $32.80
Grape Expectations Dress, Modcloth.com, $59.99
Bow Business Dress, Modcloth.com, $61.99
Scene of the Crimson Dress, Modcloth.com, $77.99
Nanette Lepore Glass Heart Sheath, Saks, $138.99
Milly Asymmetric Ruffle Dress, Saks, $141.99
Little Navy Tea Dress, Anthropologie, $188.00
Strapless Colorblocked Dress, BCBGMAXAZRIA, $298.00
Generally the cheap dresses (like Forever21) aren't as well made and don't fit as well, but if you order now you have time to try them on and tailor them if you need to. Forever21 can be very slow with shipping, also. Modcloth has flat rate $7 shipping and free returns, but because they use a huge variety of designers you HAVE to measure yourself and use the measuring guide on each garment to make sure you'll order the right size, and again, order early enough to tailor if need be. That last dress, by the way, I tried on for a spring formal and LOVED it. It's expensive, but you'll look like a million bucks in it and you'll be able to use it for other events later (that's true of most of these dresses). By the way if you have a Target near you it's a great place to find shoes.
I looooooovvvveeeeee online shopping, even if it's not for me.