Do you mean in real life, or in the game?

The prize money in the game (if you're using the original DAT file) is very inaccurate, but I'm sure it will be updated in future versions.

In real life, the most valuable races in the UK are:

Epsom Derby (£1,250,000)
King George VI & QED Stakes (£750,000)
Juddmonte International Stakes (£500,000)
St. Leger (£475,000)
Eclipse Stakes (£425,000)
Epsom Oaks (£375,000)
July Cup (£360,000)
Golden Jubilee Stakes (£350,000)
Prince Of Wales's Stakes (£350,000)
Champion Stakes (£350,000)
2,000 Guineas (£330,000)
1,000 Guineas (£330,000)

Worldwide (in US dollars), the most valuable races are:

Dubai World Cup ($6,000,000)
Breeders' Cup Classic ($5,000,000)
Dubai Duty Free ($5,000,000)
Dubai Sheema Classic ($5,000,000)
Japan Cup ($4,770,000 approx)
Melbourne Cup ($3,860,000 approx)
Breeders' Cup Turf ($3,000,000)
Prix de l'Arc de Triomphe ($2,550,000 approx)
Takarazuka Kinen ($2,520,000 approx)
Epsom Derby ($2,350,000 approx)
Hong Kong Cup ($2,320,000 approx)
Cox Plate ($2,290,000 approx)
Golden Slipper ($2,290,000 approx)

You can get more info at www.horseracingintfed.com

Mike.