Hmm this is hard because some stores don't have their spring lines out yet. Can you post some pictures of dresses that you like so we can try to find cheaper options? How fancy does it need to be? Would you consider a skirt + top combo?
Nordstrom has some cheap dresses in their Junior's department that could work for a fancier event:
AE has some sundresses if it is more casual: also comes in blue also comes in red in pink or navy
There's always Delias:
And Anthro has some things on sale for under $100 that could work for spring. If I recall correctly you wear a small size? in 0 and 2 in a 2 in a 4