Assuming combo is a select element
var combo = document.getElementById("combo");
Have you tried getting selectedValue by doing
var selectedValue = combo.options[combo.selectedIndex].value;
or
var selectedValue = combo.options[combo.selectedIndex].text;
Edit - unless that's fine and I misunderstood the question, this line may be incorrect, as well; you were missing 's wrapping value="+selectedvalue+" - should be value="'+selectedValue+'"
var row='<html:hidden property="listaPrezzoForm['+combo+'].idUAServizio" value="'+selectedValue+'"/>';