There really isn't a rhyme or reason. We fly from PIT to MCO and tickets $200 and under(including tax) is a good price. Tickets did drop to $69/each way and that is a very good price at $159.40 per person. We also fly from PIT to BOS. I just bought my dh a ticket for Weds. for $39/each way! Right now, a few days later that same day is $158/each way. You just need to check often...at least twice a day. Southwest has no change fee, so if the flight you do choose goes down in price, you can change to the cheaper flight, even if it's the same flight you already have. The flight to MCO I purchased recently changed 4x in one day. First it was $82/each way and I went to purchase but waited to long with it in my cart. 20 minutes later the fares for my dates went up to $149/each way and I cried. A few hours later they dropped to $89/each way...then, an hour later to $69/each way. I snagged them at the $89 price and changed them to the $69 price an hour later.
What dates are you looking at?