Originally Posted by
Antarius
If OP is switching from a non-peak to peak time, the fare could definitely increase significantly, even using historical prices.
Agree that OP should try again and see. The fact that they have two disparate prices tells me that all is not kosher with the replies they got.
True, some fares [and very common on the cheaper ones] are filed with valid periods so moving outside of that could cause the jump. It certainly shouldn't be repriced as a one-way as suggested above though. If OP shares the fare codes, route, flight and ticketing dates then we can look on ExpertFlyer to see if that's the issue.