If you search on this website under the tour operators, most of them say how much each tour is. So do a sample itinerary for yourselves and figure out approximately what tours you will be going on and how much that will cost. Then factor in that your meals, assuming you aren't eating at the kiosks or Dos Cocos every night, could be in the range of $20 US. and whatever lunches and brekkies you might eat out, $7-10. Then figure out how many alcoholic drinks you expect to average a day at $5 a piece. (Sure you can drink for less and may well do so but there's tipping and all sorts of things, the money just seems to slip away when imbibing).
In any event, for myself, I figure out the activities that I expect to pay for and then $50 US per day on top of that. Some days I spend less than $50 and others a lot more. My booze factor can be high . . . first you think you will just have one beer but that tasted like another and then you go home and get ready for dinner. You might have a drink or two in your room while getting ready, then you'll have a drink or two at dinner, then it's time to go to Jaquar's or Iguana's where you will think you will only have a couple more . . . and then it's 4 am and you don't have any money left . . .
Of course, if you are a teetotaller, you can easily do it on less than $50/day.
What do you usually spend when you are on a vacation? I haven't found that my budget differs greatly from place to place, unless we are trying to compare apples to oranges (ie: Grand Cayman)