At 4:50 am on September 3, 1967, horns blared and loudspeakers proclaimed the switch. From that time onward (actually at 5:00 am), everyone in Sweden would drive on the right side of the road.
Countries that mandate right- and left-hand side traffic:

Where are we going? To the cost of the switch.
The Story
Used to left-side traffic, 83% of all Swedes voted “No” when asked if they wanted to change to the right side. The “lefties” even said, “Do you want to see your mother killed?”
But the politicians did not listen. Expressing concern that almost everyone including Norway and Finland drove on the right side, that tourists were confused and accident prone, and that most cars already had left-hand drive because right-hand was tough to get, they ignored the vote.
So the planning for Högertrafikomläggningen (right-hand traffic inversion) or H-Day began. They even had a song competition. This catchy tune won:
Meanwhile, imagine all the details. Roundabouts could no longer be counterclockwise, 360,000 road signs had to be changed, the direction of one way streets was reversed, bus doors had to open on the other side, off ramps became on ramps. And among drivers, they would have massive disorientation.
Below is one of the signs and a street in Sweden at the time of the changeover.

Our Bottom Line: Cost
Defined economically as sacrifice, the cost of the Swedish traffic switch affected government, businesses and households. For government, time and money that could have been used elsewhere were allocated for writing and distributing pamphlets, publicity, the song campaign, and the changes the entire transportation infrastructure required. Though I found nothing specific about the time and money cost from businesses and individuals, we can hypothesize it too was considerable. When traffic patterns change, so too will retail habits. Finally, I do wonder about the cost of national confusion.
One benefit? Fewer traffic accidents during 1967 because everyone drove more carefully.
My sources: It was such a pleasure to hear the Swedish story from 99 percent invisible while walking this morning and then I read about it at Volvo and Wired.