What you should do is use the estimated charges from the financial aid web site:
Boston University - Office of Financial Assistance - Cost of Education
It lists the estimated billed expenses for living on-campus as $48,468 for the year. This will break down to about $24,234 a semester. The charges could increase or decrease if you decide to take a more expensive meal plan or select a more expensive room. But, this figure will be a good start off point.
Then, you will want to subtract all the financial aid you are receiving, except for work study from the total. For example, if you received $20,000 in financial aid for the year, then you will want to subtract $20,000 from $48,468. This leaves $28,468 that you will need to finance. Since the Tuition Pay is over ten months, you will divide the $28,468 by ten. You are left with a monthly payment of $2,846.80 a month. The only other thing you need to pay is the $60 enrollment.
You can always change the monthly payment at any time by contacting Tuition Pay. You may want to adjust payments if, for example, you receive more financial aid or if you want to take out a loan to defer upfront payments to chose the option of making payments over time.
Hope this helps.