select *
  from orders o
 pivot (
  sum(val_order) cnt,
  sum(val_order * price_order) amount
  for oper in (1 buy, 2 sell));

   BUY_CNT BUY_AMOUNT   SELL_CNT SELL_AMOUNT
---------- ---------- ---------- -----------
        18       1080         21         990

select 100 as price_order, 10 as val_order, 1 oper from dual union all
select  10 as price_order, 10 as val_order, 1 oper from dual union all
select   5 as price_order, 10 as val_order, 2 oper from dual

select 100 as price_order, 10 as val_order, 1 oper from dual union all
select  10 as price_order, 10 as val_order, 2 oper from dual union all
select   5 as price_order, 10 as val_order, 2 oper from dual

select 100 as price_order, 20 as val_order, 1 oper from dual union all
select 100 as price_order, 10 as val_order, 2 oper from dual union all
select  10 as price_order, 10 as val_order, 2 oper from dual

select price_order,
       sum(sum(decode(oper, 1, val_order))) over(order by price_order desc) sum_val1,
       sum(sum(decode(oper, 2, val_order))) over(order by price_order) sum_val2
  from orders o
 group by price_order
 order by 1;

PRICE_ORDER   SUM_VAL1   SUM_VAL2
----------- ---------- ----------
         10         18          1
         20         18          1
         30         13          6
         40         13          8
         50         13         13
         60         13         19
         70          9         21
         80          5         21
        100          3         21

9 rows selected.

select min(price_order) +
       nvl((max(buy_amount) - min(sell_amount)) /
           nullif((max(buy_amount) - min(sell_amount) + max(sell_amount) - min(buy_amount)), 0) *
           (max(price_order) - min(price_order)), 0) market_price
  from (select t.*
               , row_number() over (order by case when buy_amount >= sell_amount then buy_amount end, price_order desc) rn1
               , row_number() over (order by case when buy_amount <= sell_amount then sell_amount end, price_order asc) rn2
          from (select price_order,
                       sum(sum(decode(oper, 1, val_order))) over(order by price_order desc) buy_amount,
                       sum(sum(decode(oper, 2, val_order))) over(order by price_order) sell_amount
                  from orders o
                 group by price_order) t)
 where 1 in (rn1, rn2)
 order by price_order;