It would come from the men's events. The men would have to distribute the same funds over greater numbers of riders. If the women suddenly had equal prizes, promoters would be forced to jack up their entry fees as they do to the top level men's events. The already low attendance at women's events might be further reduced by increasing entry fees. If the goal is to encourage new women racers, this might not have the desired effect in a situation where there often are not enough female competitors to even have two groups of women racing.
Government intervention is almost always a bad solution.
I think a better solution would be to generate interest in women's racing with sponsors, as was done with women's tennis. Education is the key. When promoters realize that women's races are interesting to sponsors, they can sell this idea to the sponsors. When the sponsors are convinced to put up equal money for women, you have a winning solution.