Hi, The answer to your question is going to depend on several factors.
For instance, what country and which school are your preteens considering? The answer to that question gives you a starting point for research on airfare, accommodations, tuition, and immigration policies (and associated fees).
What subject(s) will they be studying, and which academic supplies does the school require students to provide for themselves? The answer to that question gives you another set of expenses to investigate.
Will food be included in the school's tuition or in the accommodation package? If not, you'll need to factor that expense in as well.
Consider also international health insurance. This expense will vary depending on your preteens' health and the coverage you seek for them.
What about tourism or extracurricular activities for the kids? That's another area of expense you'll need to consider, as well as phone and computer/Internet charges.
Some international schools will have estimated budgets already calculated for families. Check with the school's enrollment office to see if the school your preteens are considering offer this information.