Torres del Paine
The nearest town is Puerto Natales, a roundtrip bus fare from there equals to 15.000 Chilean pesos (about 37 US-Dollars, December 2009). It is fairly possible to get rides between the park and Puerto Natales by hitching. You'd probably change rides in Cerro Castillo (in any direction, at least via the old gravel road), where the Puerto Natales to Torres del Paine road meets the one to El Calafate.
Hitching out of the park, there are two possibilities: Either via the old gravel road for which you'd stand on the Laguna Amarga porteria/guarderia or via the new sealed road, for which you'd stand on the park administration center in the southern part of the park. The former road is used by Puerto Natales buses, the latter apparently more usually by private traffic.