The 29-year-old reality star's friend Ben Baller, who designed the ring, revealed Kardashian paid about $325,000 for the seven-carat diamond sparkler in an interview with E! News.
"She loves it," Ben Baller said of Blac Chyna. "$325,000 is pretty close to what [Rob] paid, but it could have been a lot more because of the clarity. It's hard to manipulate that kind of radiance on a diamond."
"[Rob] paid for it himself," he revealed. "He called me [about a week ago] and he was like, 'I have some ideas' ... He said, 'What can do you for me?' I said, 'Trust me.' And he agreed."
"What they have is genuine," the jeweler added. "It happened fast, but it's real ... [Rob is] genuinely happy again. The old Rob is back. He's a prankster and he's happy and he's funny. He's back to the old Rob and I'm glad to see it."
Kardashian and Blac Chyna got engaged Monday and celebrated with Blac Chyna's mom at a strip club the same night. The couple announced their engagement by posting a photo of the ring with the caption "YES !...!...!" the next day.
"Congrats to @RobKardashian & @BlacChyna on getting engaged," Ben Baller wrote on Instagram. "Rob, I have never seen you this happy before and I've known you since you were a teenager."
"I'm glad you came to your day 1 homie to get this massive [ring] for your lady," he added. "You know I don't play when it comes to diamonds. God bless #7Carats #VVS #IFANDCO."
Kardashian and Blac Chyna were first linked in January and have been inseparable since. The reality star's family has yet to comment on the engagement, although Blac Chyna's ex-fiancé, Tyga, said he is happy for the couple in a series of tweets.