Which was the first grand slam tournament to offer equal prize money to women and men?

According to the WTA website (refer to the link below):

"1973 - Billie Jean King founded the Women's Tennis Association, uniting all of women's professional tennis in one tour. The WTA was borne out of a meeting held in a room at the Gloucester Hotel in London the week before Wimbledon; the US Open, for the first time, offered equal prize money to the men and women."