Castillo, 22, struck in the 26th minute after combining passes with Daniel Luduena. The goal ended Toluca goalkeeper Hernan Cristante's 772-minute shutout streak. But Toluca responded a minute later with a goal by Edgar Duenas. Toluca, which needed only a tie on aggregate to advance thanks to its superior regular-season record, made it 2-1 shortly before halftime on a goal by Chilean forward Hector Mancilla that was set up by Antonio Naelson.
Cruz Azul tied Atlante, 1-1, on Saturday in their second-leg semi to advance 4-2 on aggregate. Cruz Azul's Cesar Villaluz opened the scoring in the fourth minute and Venezuelan Giancarlo Maldonado scored the game's only other goal three minutes later.
Cruz Azul, which finished runner-up to Santos last season, won its last title in 1997. Toluca last won the title when it lifted the 2005 Apertura crown.
Castillo played in all 17 regular-season and all four playoff games for Santos in the Apertura 2008.