taxi
Both the Altamirano family at Base Camp and the refugio Huayna Potosà can call a taxi for you. The cost is Bs. 150 to 180.
by bus
Catching a bus from Zongo Pass will not be a problem. remember: Buses who enter the Zongo valley in the morning should return in the afternoon, there is no other exit. They might be full of passengers and goods, but will take you. Try to ask at the refugio or at Base Camp for schedule.
hitchhiking
Hitchhiking is often your only budget choice when getting back to La Paz/El Alto, but there is little traffic. Llama shepherds say it is easier to walk 4 km to the police checkpoint, and hitch from there. Drivers ask for about Bs. 15 per person for the ride. Be sure to have camping equipment if stranded.