You can use either US $ or Belizean money. It's set at 2 BZ to 1 US. There are no debit card facilities or instabank access. If you pay in US $, you will mostly likely receive any change in BZ $. As for what your budget for a day could be, well, that depends on how much and how well you guys want to eat and drink.
If you are used to all inclusives, there's a good chance that you are used to drinking domestic product anyway, am I right? The local booze is not so much, otherwise it's around $5 a drink same as at home. Some places charge more for a Belikin than others.
There are any number of places to eat, a few quite expensive, many reasonable and some very inexpensive so it all depends on what you guys like or how flexible you are willing to be. Will you get a condotel and be able to whip up your own breakfasts and snacks, have your own beers and blend your own drinks? Will you stay at Ramon's or the Sunbreeze and have to eat breakfast out all the time? Will you stay somewhere that will require you to need a cart or to take the water taxi to town all the time? Etc. etc.
I'm pretty sure there was thread a few months back that covered this quite well. Does anyone out there remember it?