To buy one
Around 100 USD without chips, which have to be purchased separatelly.
To build one
Much, much more. Usually ~100 USD just for one bare PCB to be made + other components and ofc, your valuable time. But fun factor = 100% if you're going to build just one or two, and if you already have all the equipment needed.
The best way to build Klondike is actually to buy one :). Manufacturers are building this at a quantity, so they are receiving discounts from companies delivering components and PCBs and from assembly houses. And if you're not owning tools like Reflow oven, oscilloscope, multimeter, loupe, hotair station and many others, your only option is to buy one, because purchasing those tools would be too costly (and believe me, I know what I'm talking about).
Only one big manufacturer is/was actually manufacturing K16 - Liquid Synergy Designs Inc.
There were couple of others, but were unable to solve hw and firmware issues, so they've bailed out.
So if you would like to buy Klondike, head to www.liquidsyndesigns.com.
And to be sort of neutral, if you're producing Klondikes, let me know at drop@projectklondike.org or send PM to zipiju at bitcointalk and I'll add you here.