[monolithed]

Что же вы нагородили то?

<script type="text/javascript">
window.onload = function(){
    var array = [['1501', '1504', '1507'],
                 ['1502', '1505', '1508'],
                 ['1503', '1506', '1509']];
    
    var select1 = document.getElementsByTagName('select')[0];
    var select2 = document.getElementsByTagName('select')[1];
    var input1 = document.getElementsByTagName('input')[0];
    var span = document.getElementsByTagName('span')[0];
    
    select1.onchange = function(){
        select2.length = 0
         for(var i=0; i<this.length; i++){
             var options = document.createElement("option");
             options.innerHTML = array[i][this.selectedIndex-1];
             select2.appendChild(options);
             span.innerHTML = select2.options[select2.selectedIndex].innerHTML;
             input1.checked = false;
         }
    };

    select2.onchange = function(){
         for(var i=0; i<this.length; i++){
            span.innerHTML = this.options[this.selectedIndex].innerHTML;
        }
        input1.checked = false;
    };

    input1.onclick = function(){
        if(span.innerHTML>0){
            span.innerHTML = (this.checked == true) ? 1500 + parseInt(span.innerHTML, 10) : 0;
        }
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};
</script>

<style type="text/css">
select, input {
    float: left;
}
</style> 

<form name="forma1" action="" method="post">
    <select>
        <option selected="selected" disabled="disabled"  value="0">- - выберите - - </option>
        <option value="1">Москва</option>
        <option value="2">С. Петербург</option>
        <option value="3">Челябинск</option>
    </select>
</form>

<form name="forma2" action="" method="post">
    <select>
        <option selected="selected" disabled="disabled"  value="0">- - выберите - - </option>
    </select>
</form>

<form name="forma3" action="" method="post">
    <input type="checkbox" />
</form>

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