Man, I'm sorry, but it's unanswerable -- at least by me-- but I can tell you a few areas of more or less fixed costs that would help you work
out the arithmetic.
1) What will the education costs for the lads be, per annum?
2) Estimated insurance
costs (travelers and boat) be? Also, dental? anybody having orthodontia?
4) Maintenance? In the old days, with older boats, be used to break something on almost every passage
, never anything too shocking, except for the dismasting
5) How much do you want to hold back for emergencies? Upgrades? Maintenance?
6) How much do you spend now for food
and drink? You'll probably spend close to that out cruising, but it is one place you can cut some costs?
7) Do you require a watermaker
machine? [popular, but most places there are laundries, or people you can hire to wash, dry, & fold your clothes]
8) Inland excursions?, tours? car hire? concerts?
I'd suggest you cost out your wants, at today's prices, and make a spreadsheet or at least a list. Other people's expenditures may provide a guide, but everybody's different, too.
Good luck with it, it is a big project