I think about this a lot. I'm not talking about travelling on holidays.
I always follow my wife on her business trips. Sometimes they end up on school holidays so it's more expensive.
Up til now, I've been following her to KL and France.
I go to KL around once or twice a month, the cost is around $60-$120 depending on how late her notice is.
Sometimes we buy my ticket and her business trip gets cancelled and we may forgo my ticket. This has happened at least 3 times. I fly on budget airline so there's no way to change the dates, and there's no way I'm going to fly alone just to utilize the ticket. That's just throwing good money after bad money.
And I've followed her to France once and we're likely to go again soon. Cost for us to fly to France is usually around $2000. This includes my flight and ......