[melky]

<select name="cena" id="test">
<option value="selectsize" disabled>выберите размер</option>
<option id="block1" value="1680">1/60</option>
<option id="block2" value="3360">2/60</option>
<option id="block3" value="4939.20">3/60</option>
<option id="block4" value="6585.60">4/60</option>
<option id="block5" value="8064">5/60</option>
</select>

<div class="block1">1</div>
<div class="block2">2</div>
<div class="block3">3</div>
<div class="block4">4</div>
<div class="block5">5</div> 

<script src="http://yandex.st/jquery/1.6.0/jquery.min.js"></script>

<script>

var divs = $( "div[class^='block']" ).hide();



$("#test").bind('change',function(){

         divs.hide().filter("div."+this[ this.selectedIndex ].id ).show();
        
});

</script>