[lev]

можно сделать одним запросом

select firstonly NewPrice from RetailItemPriceHistory
    order by NewPrice
    where  RetailItemPriceHistory.ItemId            ==  itemId
          && RetailItemPriceHistory.InventLocationId  ==  InventLocationId
          && RetailItemPriceHistory.TransDate         >=  "05.01.2011"
          && RetailItemPriceHistory.TransDate         <=  "20.02.2011"
          && RetailItemPriceHistory.NewPrice
join firstonly NewPrice from RetailItemPriceHistory2
    order by TransDate desc
    where  RetailItemPriceHistory2.ItemId                ==  RetailItemPriceHistory.ItemId
          && RetailItemPriceHistory2.InventLocationId  ==  RetailItemPriceHistory.InventLocationId
          && RetailItemPriceHistory2.TransDate           <    "05.01.2011";

return min(RetailItemPriceHistory.NewPrice, RetailItemPriceHistory2.NewPrice);