I want to travel from La Paz to Santiago de Chile by bus and was wondering which company to use and how much it would cost. I know its a 24 hour bus ride.
It's actually longer than 24 hours. The most common route is from La Paz through to Iquique, and then down to Santiago.
The Santiago->Iquique leg alone takes 24 hours. I can recommend either Pullman (we used them for that) or Tur Bus (used them for other shorter trips). I blogged about it as well.
From Iquique to La Paz it took us 16 hours. However, several hours of this was waiting for the Bolivian border to open in the morning, so depending on what time of day you travel, it may be faster. I blogged about this too. We picked some random local bus company we found in Iquique, and it was a rough, cold ride, but it was only about US$15, so at about a dollar an hour, it was pretty great value.