To get a new dress or not

So I have a wedding next Friday at the Castle on the Hudson. I think its going to be a pretty glammed out event. It is my mom's best friends daughter (who I am also close with and have known since I was born) and they always throw really fancy parties.

Anyway I really feel I should wear a long dress or at least calf length...what do you think? I really dont feel like spending the $$$ for a new dress though and there is nothing for under $100 anywhere I have looked so far. The only other dresses I have are very nice, but cocktail length, like knee length. What should I do, hunker down and get a new dress or just deal with what I have?

Also DH hates when I go shopping and complains about it for days, it is really frusturating to deal with him and he has been saying for weeks to wear a dress I have.

PS - My mom is wearing a formal long gown and my younger sister has a calf length dress from white/black market