Do this:
double discount = 1.0;
string membership = textBox4.Text.Trim.ToLower();
switch (membership)
{
case "bronze": discount = 0.10; break;
case "silver": discount = 0.20; break;
case "gold": discount = 0.30; break;
}
Then multiply the original price by (1.0 - discount) to get the new value.