Can someone quickly tell me if I'm on the right path?

I'm manually entering a pile of invoices to finalise end of month
payments but because the supplier changed the format of the invoices I
have to manually enter the cell values such as:

RENT Variable Outgoings
=((100+200)*1.1) + ((250+900)*1.1)

this needs to be done as the GST is calculated on each individual item.

I was trying to make a public function but I can't figure out the
input.

What I want to do is in a cell enter the following
=igst(100,200,250,900) and the function will return the correct result.

But I get errors.

This is what I've done.

-------------------------------------------------------------------------------------
Public Function igst(range) As Double

Dim sumarray As Double
igst = 0

sumarray = DSum(igst) * 10
gst = sumarray

End Function
-------------------------------------------------------------------------------------

Maybe a for loop would work?