monthly_costs_sources = costs.groupby(['month', 'source_id']).agg({'costs':'sum'}).reset_index()
new_for_romi = pd.merge(new, monthly_costs_sources, left_on=['first_order_month', 'source_id'], right_on=['month', 'source_id'])
new_for_romi['cac'] = new_for_romi['costs'] / new_for_romi['cohort_users']
new_for_romi.pivot_table(
        index='first_order_month',
        columns='cohort_lifetime',
        values='cac',
        aggfunc='sum')