Good evening everyone,
I have bought some (uhm well actually quite a lot) big crates (the middle size) and they do drop a ship with 12.5% probability (per medium crate).
However, they will give you lower tier ships first. So whatever tier 5 -7 ships you do not have from the list, expect to get them with a higher probability. Only when you start having all the tier 5 - 7 common ships, you will start getting rarer ships like the kamikaze, Gremy and tier 8/9 ships.
In terms of 'moneys worth' you will get ships, coal and doubloons for the value of the money spent. Flags and camos are 'on top'.
Let me know if you want more stats.