I've got the same dilema as you - all the dresses i looked at were way too expensive for something i was more than likely only going to wear once, i ended up finding a really nice top for $50.00 in one of the maternity shops here -just have to hope that i still fit into the black skirt i have hanging in my wardrobe by the time the wedding comes around (its got an elastic waist - it fits at the moment).

Goodluck, hope you find something!!