Data From Database Is Displayed On Listbox And Sum Of It On A Textbox

I'm having a problem with my software.

This is are the parts of my system where my problem occurs:
- Search Button
- List Box1
- TotalTextbox
- Button
- SQL Database with a Column named "Amount" and it only contains numbers Datatype is System.String

So, basically a user clicks the Search button and searches for a data in the SQL Database, and the result of the search displays in different Listboxes including the "Amount" Column from the database, which has it's own Listbox named "Listbox1". the Listbox is now filled with numbers.

Goal: The goal is to get the Sum of all the "Numbers" Displayed in the Listbox1, and the Total is then Displayed into a Textbox via using Click event or automatically.

The Code:

Dim sum As Double
For x As Integer = 0 To ListBox1.Items.Count - 1
sum += CDbl(ListBox1.Items(x))
Next
TotalTextBox.Text = sum.ToString

And i got this Error: Conversion from type 'DataRowView' to type 'Double' is not valid.

I ended up with this..

Dim sum As Double
For x As Integer = 0 To ListBox1.Items.Count - 1
sum += Val(ListBox1.Items.Item(x).ToString)
Next
TotalTextBox.Text = sum.ToString

But the values are not adding up, the total is always at 0. i've been searching for hours now from the net to a better way of doing this.

Here's a quick Flow on what im trying to accomplish here. Record from Database -----Displays on---> Listbox1 -----User Clicks--> Button1 ----Displays Total sum on----> Textbox1