分段计算函数怎么写？

(2500-1000)*60%=900而不是600,所以结果应该是1350

Sub test()
    Dim str    As String
    Dim str1   As String
    Dim str2   As String
    Dim i
    Dim a
    Dim b
    Dim r

    str = InputBox("请输入分段信息,用逗号分隔", , "100,1000")
    str1 = InputBox("请输入计算比例,用逗号分隔", , "0.5,0.6")
    str2 = InputBox("请输入需要计算的数值", , "2500")
    a = Split(str, ",")
    b = Split(str1, ",")
    For i = 0 To UBound(a)
        If i < UBound(a) Then
            r = r + (CInt(a(i + 1)) - CInt(a(i))) * b(i)
        Else
            r = r + (CInt(str2) - CInt(a(i))) * b(i)
        End If
    Next
    MsgBox r
End Sub