Hallo,
I haven't been to Ojos del Salado but a freind of mien who has been there told me he had paid 200$ (year 2000 and off season / May) Zosimo Maravi, Peruvian Tour, calle Infante No. 530 Copiapo, FONO : 52-216-309 y tambien 52-230-602, CELULAR : 09-447-7748, FAX : 52-217-604 y email : copaoro@tnet.cl to take their team of 2 from Copiapo to Laguna Santa Rosa (?) and the day after, from there to Laguna Verde / Refugio Atacama. Note: my freind is of Chiliean origins and is fluent in spanish. He is not sure the price and/or the address is still valid but you can at least try...
Good luck for both organizing and the climb,
Ario