To help you further, just as an example, flying rountrip (RT) from PHL to MCO December 7 (Tues) and returning one week later (December 14), the cost on US Airways or AirTran is approximately $149 for non stop flights.
If you left Sunday, December 5 and returned on Sunday, December 12 on these airlines, the cost for one adult passenger would range from $179 to $254.
On Southwest Airlines, December 7 (Tuesday) to December 14 (Tuesday) could be as low as $139, depending on the flights. For Sunday, December 5, and return on Sunday, December 12, the fare for an adult could be as low as $189. Of course, all of this is just relative and depends on availability and whether you make the reservations online, or through the airline reservation system, or if there are special promotions, etc.
So, for the days chosen your original question of is Sun to Sun more expensive, it is for the days I chose at random for Philadelphia to Orlando RT. But, this could change, depending on many factors. Hope this helps as a very general answer to your basic question. But, you still might want to consider using a
travel agent, if you are not sure about making reservations yourself, or for many other reasons.