Instead of using CheckedListBox, you could have used 5 checkboxes and that would have made your life easier. See below -
Add 5 checkboxes say c1,c2,c3,c4,c5 and add event handlers for CheckedCHanged Event
private void c1_CheckedChanged(Object sender, EventArgs e) {
if(c1.Checked)
txtTotalAmount.Text = double.Parse(txtTotalAmount.Text)) + 0.5;
else
txtTotalAmount.Text = double.Parse(txtTotalAmount.Text)) - 0.5;
}
Change the amount you want to add/subtract in each checkbox event and this solves what you are looking for.