Sum decimal Prices together in one total

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C#

I have a list of prices(R199.99, R999.99, etc) chosen from a combobox that displays the selected prices into a LISTBOX...now I want to sum up all those prices using a btnCalculate and shows the anwer in a txtAmount.text

I've used the following code so far:

Dim Amount as Decimal
If Amount = lstPrice.Items.Count then
txtAmount.text = "R" + (Convert.Tostring(Amount))
End If


Please Help