Aqui está uma opção com uma
union all
declaração:select idoffer, min(price)
from (
select idoffer, price from tablea
union all
select idoffer, price from tableb
) t
group by idoffer
union all
declaração:select idoffer, min(price)
from (
select idoffer, price from tablea
union all
select idoffer, price from tableb
) t
group by idoffer