At 6,962 meters above sea level, Argentina’s Cerro Aconcagua  (viewed here from Chile’s Parque Nacional La Campana ) is the highest peak not just on the South American continent but in all the Americas. It attracts day hikers, trekkers, and climbers all year, though the approaching summer is most popular (in fact, the official season began on November 1, and will continue until March 15; there is a separate winter season with restrictions).
Everybody who travels from Chile to Argentina over the Libertadores pass catches at least a glimpse of the "Roof of the Americas" and its new visitors center, as Argentina’s Ruta Nacional 7 is its southern boundary. This is where visitors get their park permits, but it’s not quite that simple because customs and immigration - for both Chile and Argentina - are a few kilometers down the road at Los Horcones. It’s necessary to enter Argentina officially before entering the park.
Conditions and fees for visitors to Aconcagua are available at the official website of Parque Provincial Aconcagua , which is not a national park; rather, it falls under the jurisdiction of Mendoza province, which charges foreigners substantially more for park access than in does Argentine nationals. In the December 15 to January 1 peak, for instance foreign climbers pay 1800 pesos (about US$475 per person) for a permit, while their Argentine counterparts pay only 600 pesos. Seven-day trekking permits, to the main base camp, cost 400 pesos (US$105) for foreigners, 130 pesos for Argentines; three-day permits are 210 pesos (US$55) for foreigners, 70 pesos for Argentines.These fees are payable only in the city of Mendoza at the Secretaría de Turismo (Avenida San Martín 1143, 1st floor), but intending climbers and trekkers can download the expedition permission form as a PDF .
Foreign day hikers now pay 75 pesos (about $20) except for the short hike to Laguna Horcones (US$2); these fees may be paid at the park visitor center on the highway. This is the first season in which there has been any charge whatsoever for Laguna Horcones, which offers excellent glimpses of the peak even for those who never get closer. In the past, this has been a doable day trip organized from Santiago but, at present, long overdue road improvements on the almost equally scenic Chilean side is making the trip over the border a slower experience.
On the way back to Santiago, by the way, all immigration and customs procedures - for both Argentina and Chile - take place on the Chilean side of the border.
It’s worth adding that, while Aconcagua’s summit is literally a walkup - there is no technical climbing - the altitude and weather make it one of the world’s most dangerous peaks, and climbers die almost every season. Prime physical condition is essential for anyone even dreaming about the summit and, even then, they have to recognize when to turn back.