for the US market 2 stroke 30, the 2cyl 30 84-86,87-02 was a 3 cylinder,the c30 was imported from 89-97.
so in various versions roughly from 1984 to 2002.
there is no average pricing.
if you're looking to power a boat with this kind of engine i would look at the 2stroke nissan line.. they made up to a 70hp 2cyl engine that was very light weight and very powerfull! if you can find one its about 1800 or so..