Air New Zealand already offers the Skycouch, in which an additional portion of the seat folds up to make them wider and you can lay across all three seats: https://www.airnewzealand.com/economy-skycouch
I've flown on it once. It's not as nice as a real bed, you can't fully stretch out without sticking your legs into the aisle, but it's a heck of a lot nicer than trying to recline in a normal economy seat.
I don't know exactly how much it cost, since someone else had bought the tickets for me, and may have used frequent flyer benefits to offset the costs. I think you basically pay the cost of each extra seat, so it's not cheap, but cheaper than upgrading all the way to business class to get seats that recline all the way to flat. If you have someone you're comfortable sharing with, then you only pay for one extra seat for the two of you.
I've flown on it once. It's not as nice as a real bed, you can't fully stretch out without sticking your legs into the aisle, but it's a heck of a lot nicer than trying to recline in a normal economy seat.
I don't know exactly how much it cost, since someone else had bought the tickets for me, and may have used frequent flyer benefits to offset the costs. I think you basically pay the cost of each extra seat, so it's not cheap, but cheaper than upgrading all the way to business class to get seats that recline all the way to flat. If you have someone you're comfortable sharing with, then you only pay for one extra seat for the two of you.