While shopping for my Boise and Seattle fares to RTB I was surprised to see Sat depart / Sun return, one day/ one stop flights were among the cheapest available. There's just no way to figure. Although not cheap they weren't outrageous.
One piece of advice I have for others is always try to book directly with an airlines. At least you'll have someone to talk to at the airport when things go wrong. If you book with a third party the airlines will do their best to disown you. If you find a good itinerary on third party visit the airline site and look there. On single carrier / group itineraries the price will likely be the same. I have booked through Expedia et al, but only if they offer a really good flight/ hotel/ car package.