We completely understand your confusion on this! Unfortunately, airlines generally make it more expensive to make changes to your itinerary than to purchase new flights outright.
When you purchase an economy class ticket, you are essentially buying a class of service within a specific price bracket, which is what determines how much your ticket originally costs. There are a limited number of tickets available within a price bracket on a given flight, which is why an economy class ticket can be sold at vastly different prices for the same flight. The price bracket also determines how restrictive the fare rules are on the ticket; usually the lower brackets carry more restrictive rules and higher change fees.
When you want to make changes to your ticket, the airline requires that we price the change in the same or a higher class of service (price bracket) as the original booking. As the calendar gets closer to the date of the flight, the lower price brackets on the flight typically close up, driving up the prices available to us. Occasionally you may see a better price offered on the web for a flight, as it will be in a lower class of service than we are allowed to change your ticket to.
Wondering why we can't offer you a lower change fee? Our hands are tied, unfortunately. The airlines require us to use the change fee as it's listed in the fine print on your ticket, whenever applicable.