Hmm for about 800 pesos it will be a bit more difficult if you're wanting something long, or if you want something a bit more dressy. But you might have some luck with some places still having summer dresses in stock.
Ciara on Gorriti usually has some things, and then if you pic up one of those shopping guides for Palermo you'll find a few more through there. There is another place called habia Una Vez that may have something in stock from the summer season.
Unfortunately I just threw out my list of places so I'll try and remember a few more.
One of my girlfriends got her wedding dress at Veronica Far for I think less than 2000 pesos, so you'd think that would mean that their other dresses would be really reasonable. Don't have the address but google it and you may find.
Para Ti online has a directory as well, and SaberaDondeIr has an online shopping guide.