Jurors at Birmingham Crown Court heard Mark Jones told other extremists he had paid "a lot" for the book, which featured a Nazi party application form.
Jones, 24, and his partner Alice Cutter, 22, both of Mulhalls Mill, Sowerby Bridge, near Halifax, West Yorkshire, deny being members of the organisation after it was outlawed by the Government in December 2016.
Garry Jack, 23, from Heathland Avenue, Shard End, Birmingham, and 18-year-old Connor Scothern, of Bagnall Avenue, Nottingham, also deny belonging to National Action between December 2016 and September 2017.
Their trial was told on Wednesday that Jones and Cutter both made comments in February 2017 in an online chat group, which included discussion of buying a book from the US.
The court heard Jones - under the username Grandaddy Terror - then posted: "£50 for a book. F*** me.
"What are you actually getting for 50? I paid a lot for an original wedding edition of Mein Kampf with nsdap application form in the back but that's a rarity.
"I would not pay that much for normal books."
After the messages were read out in court by a police officer, jurors were shown a picture - recovered by police in September 2017 - of a masked male alleged to be Jones holding a copy of Mein Kampf.
Other messages from the group chat read to the court on the sixth day of the trial included posts allegedly from 22-year-old Cutter telling a wheelchair-user to "get back in the sea where you belong".
A series of messages allegedly written by Cutter, who is said to have entered National Action's Miss Hitler contest, stated: "I discovered my least favourite demographic," before adding a disparaging word about those with learning difficulties.
The court has heard an online chat group called the "Triple K Mafia" included posts discussing "lesser races" mixing with Aryans.
It is also alleged Cutter messaged her partner asking him to return home to "make sure everything is wiped" as the Government announced a ban on National Action.
The trial continues.