Frontier will allow you to reticket the flight for a $35.00 charge. That said, it doesn't make sense to reticket a flight when the price has only dropped $35 or less. If you buy a $500 ticket and two weeks later the price is only $350.00, then paying the $35.00 fee is completely worth the effort.

Have you tried using Yapta yet? www.yapta.com It keeps track of ticket prices, including the price of a ticket you've already purchased. It works with Frontier's booking systems - you just plugg in your flight info and purchase price and it takes care of the rest.