Drill Down Functionality for Data Entry in Database
One of my favorite approaches to problems like these in the context of data warehousing usually takes the form of an agile design practice, where the granularity of a system does not need to always be "rolled up" from the lowest level right away, but can be, as stated in the title of the topic "drilled down" into, as the finer granularity details become available.

Assuming you have some table Expense with columns Expense_PK, CompanyName and TotalExpenses,  I would simply create a new table, ExpenseDetail, containing columns ExpenseDetail_PK, Expense_FK, Taxes and OtherExpenses, adding a detail record when one was available.  

The nuance of doing this is to be stalwart in maintaining the detail record properly.  Should the nature of OtherExpenses be revealed to be solely comprised of Adjustments, Dues and Assessments, it is often far better to extend the detail record with the additional atomic columns for each of the now determinant fees than to add another OtherExpensesDetail table to the mix.  On the other hand, in Canada anyway, the Taxes column would pose a more than reasonable opportunity for a TaxesDetail table, defined TaxesDetail_PK, ExpenseDetail_FK, GST, PST, so that Company A could provide additional tax breakdowns in the form of Federal and Provincial taxes, where as theoretical Company C could simply be allocated the aggregated Taxes value in the detail record if that was all that was available or were operating in a province that only applied a singular harmonized sales tax ( HST ).

That said, I feel there are many ways of going about problems like these, but this is at least one example of something that is quite versatile and works quite well for what it is.