<form method="post" enctype ="multipart/form-data" name="myForm">

<script type="text/javascript">
function checkFields(){
var website = document.forms['myForm'].website.value;
var homepage = document.forms['myForm'].homepage.value;
var duration = document.forms['myForm'].duration.value;

if(website == ""){
website = 0;
} else{
website = 4;
}

if(homepage == ""){
homepage = 0;
} else{
homepage = 20;
}

if(duration == ""){
duration = 0;
}

var total_value = website + homepage + duration;
}

// then on calculate() do something like this:
function calculate(){
var total = 100; // enter whatever you want the total to be
checkFields();
var new_total = total - total_value;
document.getElementById("showNewCalculations").innerHTML=new_total;
}
</script>

var website = document.form['myForm'].website.value;
//or
var website = document.form['myForm'].elements['website'].value;

var website = document.getElementById('myForm').elements['website'].value;

// or assuming each field has an id

var website = document.getElementById('website').value;

function calculate() {
var website = document.getElementById('website').value;
var homepage = document.getElementById('homepage').value;
var duration = document.getElementById('topad').value;

if(website == "") {
website = 0;
} else{
website = 6;
}

if(homepage == "") {
homepage = 0;
} else{
homepage = 20;
}

if(duration == "") {
duration = 0;
}
else if (duration == "3") {
duration = 50;
}
else if (duration == "7") {
duration = 50 * 2;
}

var total_value = website + homepage + duration;

document.getElementById("showNewCalculations").innerHTML='$'.total_value;

return statement here;

// eg

return something + something * something

var = someFunction();