Do you know the wives/gfs of dh's co-workers? Check what they're wearing? Also, the event's location can give you a better idea of what people will be wearing.
I had to post this gorgeous Lanvin gown. It's more than you wanted to spend but it's sophisticated and sexy in an understated way...a dress that's appropriate for a stylish wife and not a PYT who doesn't know any better (as Tamarind pointed out). With the right jewelry and shoes, you'll look like a goddess! The draping is amazing on this dress.
http://www.barneys.com/Drape Front Gown/500360231,default,pd.html