GCS: I misread your post.......when you said "per couple" , it sounded to me that multiple couples would be doing the charter.
Now the numbers make more sense.
Lets use Sunsail for demonstration purposes & BVI as the location
Christmas falls in the Peak (most expensive 12/21-1/4) season
Newer (< 2yrs) 34' monohull Boat = $510 @ day
+++ Misc - Damage Waiver, Fuel, Cruising Tax
Memorial Day falls in the Low (cheapest 5/10-6/6) season
Newer (< 2yrs) 34' monohull Boat (same as above) = $ 260 @ day
Misc the same except cruising tax is cheaper
Choose older boats = bet 2 & 5 yrs old...can cut rate 5-25% depending on co.
Look for specials & discounts = ie; Sunsail offers reef weeks
Look at Tier B companies
Look at Owner's time........you can get a discount http://www.sailonline.com/discount-charters.html
The more flexible You are the better deal you will be able to get. I try to take full advantage of offerings and it has worked out. Plus we often end up with less crowds (high priority) and cheaper airfare too.
In fact, I shop equally hard for the airfare & will not lock in boat (put it on hold) until I get flight reservations. Unfortunately, depending on your departure city and/or charter location, deals on airfare are not always easy.
Good luck.............but you got plenty of resources here to help.