How To Subtract CBOT Grain Prices
The following will shorten your code considerably:

Public Function GrainSubtraction(ByVal numberOne As Double, ByVal numberTwo As Double) As Double
    Dim Dec1 As Integer = CInt(Int(numberOne / 10.0))
    Dim Oct1 As Integer = CInt(numberOne - (Dec1 * 10))
    Dim Dec2 As Integer = CInt(Int(numberTwo / 10))
    Dim Oct2 As Integer = CInt(numberTwo - (Dec2 * 10))
    Dim ResultDec, ResultOct As Integer
    If Oct1 &lt; Oct2 Then
        Dec1 -= 1
        ResultOct = CInt(7 - (Oct2 - Oct1))
    Else
        ResultOct = CInt(Oct1 - Oct2)
    End If
    ResultDec = CInt(Dec1 - Dec2)
    Return CDbl((Math.Abs(ResultDec * 10) + ResultOct) * Math.Sign(ResultDec))
End Function


This does simple octal subtraction on the last digits of the numbers supplied and decimal subtraction on the rest.  I left the parameters and the return type as doubles but it would make more sense to have them as integers, since that's what they basically are.