Agree with last two posts- the bridesmaids usually have to buy their own dress.  It sounds like she is not paculiar about the style of the dress if you are shopping on your own.  So why not go to a cheaper store and get something you can wear that once.  Usually anything that is associated with weddings doubles or triples in price.  It should be fairly easy to find something at a reasonable price in a department store like Macy's.  Since you are the only bridesmaid, you don't have to match colors with anyone, so that should make it easier on you as well.  My thought on this is that if you really don't want to do it, tell her now so that she can find alternatives as wedding planning is stressful enough.  If you want to do it but don't want to get the dresses you were quoting, then find a cheaper dress if you are just going to wear it once.