Does anyone knows how can I get from Uyuni, Bolivia to Salta, Argentina by bus? And what the approximate cost/time will be to get there?
So there are two options.
1) You're planning on doing a Salt Flats tour, if you're going to Uyuni? Most of these run from Uyuni to San Pedro de Atacama in Chile, or vice versa, and take 3ish days. It's well worth it and you should consider that if you haven't already, as it'll make the next part really easy. Once you're in San Pedro, there are buses over the Andes to Salta, and it takes approximately 11 hours.
2) If however, you're set on going direct from Uyuni to Salta (perhaps you're only doing a day tour in Uyuni, which would be a shame but understandable if time restricted), then it's important to realise there are no direct buses from Uyuni to Salta. You'll need to complete two stages - first from Uyuni to the border town of Villazon, and then down to Salta in Argentina. Buses tend to go via Tupiz to Villazon, and are not that regular - so if you're doing a tour you'll want to check before hand what time the buses are and whether they can book you a ticket beforehand.
Then you cross the border into La Quiaca (it's like the same town, just other side of the border, different time zone too), and catch a new bus down to Salta, which takes about 7-8 hours. You walk across the border, and there are several bus companies at regular intervals down to Salta - that's the easy part of the trip - Argentine buses are also much more comfortable :)
As for cost, it varies dramatically by bus company, level of comfort, time, day of week and with the current crazy fluctuations in the $ARS, I'd be guessing. From talking to some Argentine folk in the hostel here the other day, it sounds like everything's varying tons at present. But in general, coming from overseas you will find them pretty reasonable - especially the very cheap Bolivian buses.