可以使用日期算术和条件聚合：

select product,
       ( sum( cost * datediff(day, beg_date, (case when end_date > getdate() then getdate() else end_date end) )) /
         sum(datediff(day, beg_date, (case when end_date > getdate() then getdate() else end_date end))
       ) as avg_price,
       max(case when end_date > getdate() then price end)
from t
group by product;