We all know the context in which it was said, but imagine an unsophisticated country guy falling off a cabbage truck into an unfamiliar town, where he’s new, inexperienced, gullible and easily manipulated. This was the thought behind that expression.
Sonny used it twice throughout the series. Once in the the episode “The Dutch Oven” when arranging a drug deal with Adonis, and the other time when arranging an arms deal with Eddie Kaye.
In crime and police slang, "buying radio time" typically meant to broadcast or give out information. It is a metaphor based on commercial broadcasting, suggesting that a person is "announcing" or "advertising" their business, secrets, or location to anyone listening.
Sonny used the expression in the episode “Brother’s Keeper” to explain to Rodriguez why he refused to update him on any progress reports, quipping that he might as well tell everyone, insinuating that someone from within was a leak, halting his progress.
In the episode “Heart of Darkness”, porn producer Jimmy Conway used the expression to tell Artie Rollins to bail Penny McGraw out of jail fast before she starts snitching.