The Glovers boss has praised his in-form striker, who is currently leading the League's goalscoring charts with an impressive 16 goals
Having initially joined the Glovers on loan from Carlisle United, former Bohemians striker Madden agreed to make his switch to Huish Park permanent in January and he is currently joint top goalscorer in League One with 16 league goals.
"He's the bargain of the season. He's the only one we've paid for and it wasn't much," Johnson told the BBC.
"Carlisle weren't using him and Paddy is a confidence player. We saw him play a couple of times and were willing to take a chance on him for a month.
"He did well and we tried to keep it a secret. When it came down to it, Paddy wanted us as much as we wanted him. Paddy is a natural goalscorer and when you find one of them you have to guard them with your life," added the Yeovil boss.