This was predictable though I am amazed it took so long with the Kentucky Derby having raised their purse eight years ago...
http://espn.go.com/horse-racing/story/_/id/10205653/belmont-stakes-purse-boosted-15-million
Kentucky Derby ($1 million 1996 to 2005)
http://espn.go.com/horse/news/2005/0104/1958932.html
Preakness ($1 million since 1998)
http://www.preakness.com/news-center/latest-news/preakness-stakes-purse-increased-15-million-beginning-2014
Belmont ($1 million since 1998)
http://www.drf.com/news/belmont-stakes-purse-raised-15-million