I read a PJ years ago where the couple held a RD for 25 or so at the Olive Garden in Lake Buena Vista, just outside DTD, and said it was the cheapest nicer option they found. I've never seen anyone else try this.
We're planning a rehearsal dinner at Epcot and trying to minimize other costs to make it fit. Since most of us will be park-hopping, we'll just plan for Epcot that day (and won't be charged the viewing fee) Everyone else will meet/walk in together from the Boardwalk area so that we don't have to rent a bus and can pay as few viewing fees as possible. With one of the lower priced dinners, smaller areas, and no alcohol, it works for us.
In the spirit of equality, we are looking into a few off-property places, too, but it's going to be a challenge to find anything significantly cheaper, we're pretty sure. Epcot would be really special, is convenient to our guests staying on property, and entertainment is definitely provided!
As a side note (not to you personally but to the lurkers reading this) if you are a couple with a rehearsal dinner group that wouldn't mix very well (it happens!) the bzillion dinner shows in the Orlando area usually have group rates. Most of these include, for about the rate of Disney's meals alone, a full dinner, evening's entertainment, and (cash) bar that you don't have to pay for! They're good entertainment for kids, too, and most of them have you seated where you're only side-by-side for minimum friction, particularly when combined with the attention-holding power of the show.
Not for everyone, certainly, but if you have a medieval/pirate/murder mystery/Arabian nights/etc themed event, or just want a diversion, you could consider it. Just trying to be helpful.