<form action="#">
<input type="hidden" name="bonus_hidden" value="" />
<select name="b" onchange="document.forms[0].elements['bonus_hidden'].value=document.forms[0].elements['b'].options[document.forms[0].elements['b'].selectedIndex].text">
<option value="1.000">100 bonus</option>
<option value="1.500">150 bonus</option>
<option value="2.000">200 bonus</option>
<option value="2.500">250 bonus</option>
</select>
</form>