In short
The Uyuni salt flat is the largest in the world: ten thousand square kilometres, perfectly level — so level that it is used to calibrate altimetry satellites.
From January to March a film of water covers the salt and turns the flat into a mirror. That is the image everyone is after, and it exists only a few months a year.
What it really costs
Uyuni is a town with no interest of its own: you come here to leave on a circuit, usually three days and two nights towards Laguna Colorada and the Chilean border.
Agencies charge similar prices, and the difference lies in the state of the vehicle, the sobriety of the driver and the quality of the bedding. The cheapest is not a good calculation here.
What nobody tells you
The circuit climbs to four thousand eight hundred metres and sleeps above four thousand. Arriving straight from the coast is dangerous: two or three nights in La Paz, Sucre or Potosí first.
Mirror season is also when some tracks become impassable and parts of the flat are closed. The mirror and full access do not always go together.
Nights drop below minus ten in July, in barely heated refuges. A sleeping bag is the one piece of kit that genuinely matters.