The DA* 300/4 has a (new) list price of $1400
The DA* 200/2.8 has a (new) list price of $1200
The 600/4 or 300/2.8 when still available on order had a list price like $6400, if my memory is correct.
The price of very long glass is mainly a function of diameter in mm (of its "weight").
Based on $1200/6400 price points, this formula would be:
price = $0.0917 * (f/N)^2.226
where
f is the focal length and
N its numeric aperture.
The resulting price for a DA* 400/4 is
$ 2600.-
or between twice the price of a DA*200/2.8 and DA* 300/4; and 200/2.8, 300/4, 300/2.8, 400/4 and 600/4 would all be modelled by a single price function.