In an interview with Edge Magazine, Yakuza series creator and SEGA Executive Toshihiro Nagoshi confirmed that F-Zero GX sold a total of 1.5 Million copies world wide. This is what Nagoshi-san had to say in regards to the development of F-Zero GX and the success of the entry.

Compared to us, in the big picture, we are similar. But in the finer details – their decision-making and timing – things are different, and I learned a lot from them. In short, it’s about objectivity. (…) It’s hard to describe, but when I’d say about some part of the game, “It’s okay like this, isn’t it?” they’d say, “Our company does not allow this kind of thing. Ever.” I didn’t manage to change their minds about anything. Not even once. But that’s why Nintendo has such a solid brand, even after all these years. That is why we lost the hardware war.
I really liked the Super Famicom game, and while we made a few proposals – Metroid for instance, and others – I was most confident in making a driving game because of my experience in the genre, though I’d never make a sci-fi one

Even though we’d lost the war in the hardware market, I wanted Nintendo to see how great Sega was as a company. We made lots of characters and courses, and we did the best we could for the graphics using the best technology of the time.
Even though we’d tried really hard making games for Sega hardware, they never sold too well, but F-Zero sold over 1.5 M copies worldwide. We realized the only thing we needed to admit was that Sega did not have the ability to sell hardware (laughs). That as a developer (…) we did not need to be pessimistic at all.

After it released, I got a call from Nintendo. They said they wanted to see all the source code for the game, and wanted me to explain how we’d made that game, in that timeframe and with that budget, in detail. They were wondering how we’d done it – they couldn’t figure it out. We were able to achieve something a lot higher than what Nintendo had expected.

Edge Magazine Interview 

This is honestly some pretty big news regarding F-Zero GX. It has been wildly accepted that the GameCube cult classic was a commercial failure, but if what Nagoshi-san is saying is true, this would have F-Zero GX’s sales be up there with the other entries in the series. Maybe one day we will get lucky and Nagoshi-san and his team will have the chance to work their magic again on a brand new F-Zero.