Actually, 5.7 ammo can be had vastly cheaper online if you seek to buy it in case quantity, in which instance shipping cost is spread out substantially. It can be had at about the cost of .40 S&W practice ammo if you look around... But, keep in mind that the couple of loads which are available are also effectively your SD ammo of choice--premium 9mm, .40 and .45 defensive loads often cost about a buck a shot at retail price. Many retail shops are also stocking it, at that price premium.
I'd really like a Five Seven to play with, but the damn things are too expensive for my current handgun budget.
As for .338 Lapua, I kind of take issue.... Disclaimer, I own a .338 LM rifle, and roll my own ammo to keep it affordable.
Lots of people want big boomers, and .338 LM has found a niche amongst long range practical shooters. Heck, both Remington and Weatherby (wanna talk expensive?) followed Lapua's lead and made comparable .338 super-magnum class rifles of their own--but about 10 years later... Though, as I've heard, neither has the inherit consistency with heavy bullets of the .338 LM.
As for 'need'...what? I might only need a pointy stick, but where's the fun in that