Monthly Service Calculator

This is a JavaScript method that will calculate and display the cost of any monthly service, subscription, or other items. 

                                function calculateCost(radioGroup, userEntry, responseMsg){
	var getAmount = document.getElementsByName(radioGroup);
	var userCost = document.getElementById(userEntry).value;
	var response = '';
	for(var i = getAmount.length-1; i > -1; i--) {
		if(getAmount[i].checked) {
			userAmount = getAmount[i].value;
		}
	}
	if (userCost != "" && !isNaN(userCost) ){
		userCost = parseFloat(userCost);
		totalCost = (userAmount * userCost) * 30;
		response += 'You will save $' + totalCost.toFixed(2) + ' per month!';
	}
	document.getElementById(responseMsg).innerHTML = response;
	return false;
}

/* clear old user values */
function clearData(userEntry, responseMsg){
	document.getElementById(userEntry).value = '';
	document.getElementById(responseMsg).innerHTML = '';
}

Here is an example of how it is used in an html page that calculates monthly cost of smoking cigarettes.

                                <form>
	<h3>How much will you save per month?</h3>
	<!-- Radio Buttons -->
	<h3>How many packs per day do you smoke?</h3>
	<label for="quarter_pack">
		<input type="radio" id="quarter_pack" name="smoke_radio" value=".25"/>
		1/4
	</label>
	<label for="half_pack">
		<input type="radio" id="half_pack" name="smoke_radio" value=".50"/>
		1/2
	</label>
	<label for="whole_pack">
		<input type="radio" id="whole_pack" name="smoke_radio" value="1"/>
		1
	</label>
	<label for="oneplus_pack">
		<input type="radio" id="oneplus_pack" name="smoke_radio" value="1.5"/>
		1 1/2
	</label>
	<label for="two_pack">
		<input type="radio" id="two_pack" name="smoke_radio" value="2"/>
		2
	</label>
	<h3>What is the price of a pack of your chosen brand?</h3>
	<input name="brand_price" type="text" class="textbox" id="brand_price"  onclick="clearData('brand_price', 'response_msg');"/>
	<input type="submit" class="submit" name="submit" value="Calculate" onclick="return calculateCost('smoke_radio', 'brand_price', 'response_msg');" />
	<h4 id="response_msg"></h4>
</form>