with prices as(
    select 1 as PriceCode,8.5 as sredPrice,0.75 as percent1,0.25 as percent2 from dual
)
,items as(
select 'item1' as item,1 as Part, 1 as PriceCode from dual 
union all
select 'item1' as item,2 as part, 1 as PriceCode from dual
)
select item,part,sredPrice
,(case part 
    when 1 then sredPrice*percent1 
    when 2 then sredPrice*percent2
end) as OriginalPrice
from items
natural join prices
; 
/*
А на деле OriginalPrice для первой строки должна получиться =8, а для второй =10
вот не могу вывести эту формулу
проверка:
select (8*0.75+10*0.25) from dual
*/