Does anyone have a DEFINITIVE price for a Kazakhstan transit visa for U.S. passport holders? ( at the Kazakh embassy in Tashkent in person)
Frankly, it may depend on who you are and what officer you get on the day.
It also depends on where you purchase it - it's possible on arrival if you fly in, but not at a land border.
One of the leading resources on the area, Caravanistan, observes that a 5 day transit visa is the standard, and that the "average" price is US$20.
As the commenters above observed, it varies in different places. At the embassy in Bishkek it's often rated as easier (When I was there at least) than Tashkent, but things may have changed in two years.
However, despite some travellers' misgivings over the operation of Caravanistan, their information is usually up to date, so I'd take their figure to be as close to accurate as I can find.
Good luck, and enjoy it!