<script>
function celkem(){
 var ks = document.getElementById("ks").value;
 var pocet = document.getElementById("cena").value;
 document.getElementById("celkem").value = (ks * pocet);
}
</script>

<table>
 <tr>
 <td>
 
 <input type="text" name="pr_nazev" id="nazev"/>
 
 </td>
 
 <td>
 
 <input type="text" name="ks" id="ks" onkeyup="javascript:celkem();"/>
 
 </td>
 
 <td>
 
 <input type="text" name="cena" id="cena" onkeyup="javascript:celkem();"/>
 
 </td>
 
 <td>
 
 <input type="text" id="celkem"/>
 
 </td>
 </tr>
</table>