The 38-year-old actress reportedly splashed the cash on the drug, which is legal in the state of California, for their wedding anniversary so Robbie - with whom she has daughter Teddy, five, and son Charlton, three - can smoke the substance freely within their mansion, according to rapper Big Narstie, who claims to have seen the farm whilst visiting their home last year.
Narstie - who was at the mansion to work on his 2017 collaboration with Robbie, 'Go Mental' - told The Sun newspaper: ''It was all very surreal. He phoned my booking agent and said, 'Robbie wants to do a tune'. We recorded together. He wasn't what I expected.
''He's very cool. He's very down to earth and a humble person, and he's got a good wife. I saw the present she got him for their anniversary. She got him a whole weed farm in their basement in LA. That's how you drive. It's legal innit? It's legal.''
The 32-year-old rapper's revelation comes after former Take That star Robbie recently admitted he had ''smoked a spliff'' whilst in Queen Elizabeth's residence at Buckingham Palace, where he performed in 2012 for the monarch's Diamond Jubilee Concert.
He said in February this year: ''Threw up in Buckingham Palace? No. I smoked a spliff in Buckingham Palace.''
The 'Angels' hitmaker has been open about his relationship with the drug in the past, and despite having been to rehab twice, he admitted in 2013 that he still uses the substance recreationally.
He said: ''The last time I got high was two days ago. No big drug sessions, mind, just a small amount, purely to relax.''
Robbie has also previously been addicted to alcohol and other drugs such as cocaine and heroin, but admitted that since the birth of his children, he has put his partying ways behind him.
He said: ''I don't want to drink and I don't want to take coke, I'm really pleased I don't take coke but I do think about ecstasy.''
His new album Underneath It All is out now.