function findPrice() {

var seats = document.calculator.seats.value;
var cost = 0;

if (seats < 11) { cost = 120 ; }
if (seats < 31 && seats > 10)       { cost = (seats * 12); }
if (seats > 30 && seats < 201)       { cost = ((seats-30) * 6 + 360 ); }
if (seats > 200)       { cost = ((seats-200) * 3 + 1380); }

document.calculator.cost.value=formatPrice(cost)

}



function formatPrice(value) {
var result= Math.floor(value) + ".";
var cents = 100 * (value-Math.floor(value)) + 0.5;
result += Math.floor(cents / 10);
result += Math.floor(cents % 10);
return result;
}

function filterNonNumeric(field) {
var result = new String();
var numbers = "0123456789";
var chars = field.value.split(""); // create array 
for (i = 0; i < chars.length; i++) {
if (numbers.indexOf(chars[i]) != -1) result += chars[i];
}
if (field.value != result) field.value = result;
}