How Can put simple with price

You have to convert the values from the Textbox input - which is a string - to a number.
The best way to do that is to use TryParse:

int x;
if (!int.TryParse(Textbox1.Text, out x))
   {
   // Complain about user input
   return;
   }
// x now holds the value.

Repeat that for the other value into y and then do your operation.
Finally, show the result as a string:

TextBox3.Text = (x * y).ToString();