1. The character made money selling "Radio on the Internet" -- Mark Cuban sold Broadcast.com to Yahoo for $5.7B in 1999.
2. The character recounts popping a boner when he saw his net worth cross 1B. There's some old Cuban video about watching his stock price while sitting in his boxers.
3. There's a bit in the show about the character showing both rows of teeth when he smiles. Cuban smiles like that.
I'm not sure if it was ever explicitly stated by the writers, but IIRC, Russ put "radio on the internet," while Mark Cuban first hit it big with broadcast.com.
I believe they based him off of Mark's big personality too, mocking things like "three commas" and investing in a bunch of stinkers over the last 20 years.
I didn't know that's what they had intended. I never once thought that's what they were going for.