Normally, buying in bulk saves you money when you calculate the cost per item. That's what makes Costco so successful. This is also the case with many transportation systems where you can buy a dozen tickets at once for a lower per-ticket rate. Anyway, I have found a bizarre exception to the rule:

Heathrow Express "carnets" of tickets actually cost more per ticket than buying them individually. See for yourself here. Return tickets purchased online cost £11.25 apiece. If you buy the allegedly convenient carnet of 12 tickets they cost £11.91 each.

The only advantage is that you don't have to specify your date of travel. Obviously the idea is to sell the carnets to business people who go on lots of spontaneous trips that don't give them enough time to order tickets online. This saves them the £13 walk up fare at the station. So... we're really talking about 2 different economies at once. But I'm still curious how this all works out in terms of profitability, since most business travellers wouldn't blink at the extra walk-up cost, but I'd gladly buy 12 tickets right now if it would actually save me money!

Ed Note:

The Gatwick Express also sells carnets, but they are in fact a better deal than if purchased individually: £9 each instead of £10.75. Go figure.