What were the requirements for concubinage in ancient times? How much does it cost to buy a concubine?

According to historical records, it is said that in the Han Dynasty, only those meritorious ministers could marry eight concubines, while other slightly famous families generally married one concubine, just like ordinary people, they could not marry concubines. The condition of concubinage is that the wife can only concubinage if she has not given birth to a boy for 20 years, and she must obtain her consent.