The first link I gave was to the series BT372. "Seamans Pouch's. I take it that the person you are interested in was British.
These records relate to individual seamen filed together in a pouch. These documents may include application forms, certificates, identity cards,(most with a photograph of the seaman) cessation documents and notifications of death.
As these documents relate to an individual and are catalogued as such, they should be easy to find and copy, therefore, theoretically, they should be relatively cheap. I have long given up on how TNA fix a charge for copying. You would need to ask for a quote.
The second link i gave was to the series BT382. Because these records are in blocks of 60 names, TNA would consider it research and charge accordingly. Again it appears that no fixed fee is applicable. I have heard of people being quoted £80 plus!
In all honesty. you would be advised to visit Kew yourself. you can the view both sets of documents for free and whilst there extend your research to maybe look at the logbooks of the vessels he sailed on.
If it's problematic for you to visit Kew. Let me know and I will attempt to take a look for you next time I visit. Whenever that may be.