Teď to mám
function change ()

{
  var inputA = document.getElementById ("format")
  var inputB = document.getElementById ("za_kus")	
 
  if (format == 16) var za_kus = 1100;
  if (format == 8)  var za_kus = 2288;
  if (format == 4)  var za_kus = 4752;
  if (format == 2)  var za_kus = 9658;
  if (format == 8)  var za_kus = 19580;	
	
	inputB.value = za_kus; 
}

<select id="format" onchange="change()" name="format" size="1">
  <option value="16">1/16 str. (6,1 x 8,2 cm)</option>
  <option value="8">1/8 (12,7 x 8,2 cm)</option>
  <option value="4">1/4 (12,7 x 17,0 cm)</option>		
  <option value="2">1/2 (25,8 x 17,0 cm)</option>
  <option value="1">1/1 (25,8 x 34,5 cm)</option>
</select>
<br><br>
<!--format-B-->
Základní sazba za zvolený formát:<br>
<input id="za_kus" type="text" onkeyup="change()" readonly="readonly" value="1 100"> Kč