Now it's more or less clear, we have done the same thing in our project when we use allocation matrix of magazine titles (title x title). For each title allocation can be stored on the title itself or on the titles described by coefficients in the matrix.
It's better to combine models A and C, storing initial data on some dummy Cost Center and Cost Element. Then you loop all not dummy records and multiply value of the current record (1 or nothing) by dummy cost center and cost element. The result will go to B using destination_app.
B.R. Vadim