Search Text String separated by Commas and do a Vlookup based on results

Hi All,
Thank you in advance for your help.
I have a string of invoice numbers, separated by commas. I am trying to figure out how to have a macro look in the string on column G, do a vlookup for each invoice # (separated by a comma), and then put the total in column H.

I have attached an example spreadsheet. Column G:H are what I need to look for and paste results. Columns A:B are what the vlookup references.

Thanks!

Function invoice_lookup(invoices As String)
Dim i, Str
Str = Split(invoices, ",")
For i = LBound(Str) To UBound(Str)
    invoice_lookup = invoice_lookup + WorksheetFunction.VLookup(Str(i), Range("A4:B8"), 2, 0)
Next
End Function

?

Try this:-

Sub MG15Nov12
Dim Rng         As Range
Dim Dn          As Range
Dim n           As Long
Dim Dic         As Object
Dim oSum        As Double
Dim oRws        As Variant
Set Rng = Range(Range("A4"), Range("A" & Rows.Count).End(xlUp))
 Set Dic = CreateObject("scripting.dictionary")
    Dic.CompareMode = vbTextCompare
    For Each Dn In Rng
        Set Dic(Dn.Value) = Dn
    Next

Set Rng = Range(Range("G4"), Range("G" & Rows.Count).End(xlUp))
    For Each Dn In Rng
        oRws = Split(Dn, ",")
        For n = 0 To UBound(oRws)
            If Dic.exists(Val(oRws(n))) Then
                oSum = oSum + Dic.Item(Val(oRws(n))).Offset(, 1)
            End If
        Next n
            Dn.Offset(, 1) = Format(oSum, "$0.00")
            oSum = 0
    Next Dn
End Sub

Regards Mick

Thank you all for your help! These worked perfectly. Have a great day.