I bought a 300s last year for £200. it was made in 1979 and has seen a lot of use as it used to belong to Nationals shooter.
the price was low because the blueing is worn in places but this is no reflection on the operation of the gun which is superb. very accurate, very consistant very solidly built.
in the uk they are going for about £250 to £400 depending on age and condition.
as far as i know they were still being built up to the mid 90's. the new price then was £850 -£900 according to an old ASI price list i have.
HTH Seggy