新手求助，购票金额为什么为0，这种方法写错在哪里

double calculate_price(i)
{
    double money;
    if(i > 0 && i < 30)
    {
        money = 50 * i;
        //printf("购票金额为: %lf \n",money);
    }
    else
    {
        money = (50 * i) - (i*2);
        //printf("购票金额为: %lf \n",money);
    }

    return 0;
}

int main()
{
    int a;
    int price;

    printf("请输入购买的票数：\n");
    scanf("%d", &a);
    //calculate_price(a);
    price = calculate_price(a);
    printf("购票金额为: %d \n",price);

    return 0;
}
/*

 variable 'money' set but not used [-Wunused-but-set-variable]|
 
*/