SELECT dc.dt,
       revenue * 100 / costs AS ROI
FROM
  (SELECT CAST(DATE_TRUNC('month', created_at) AS date) AS dt,
          SUM(costs) AS costs
   FROM tools_shop.costs 
   GROUP BY dt) dc
JOIN
  (SELECT CAST(DATE_TRUNC('month', paid_at) AS date) AS dt,
          SUM(total_amt) AS revenue
   FROM tools_shop.orders 
   GROUP BY dt) dr ON dc.dt = dr.dt
ORDER BY dc.dt;