i should add that money is usually the driving factor.
There was a Formula 3 driver who won the title back in the day on a tiddly budget. Though he had been driving for what was then the top F3 team of the day - he was signed based on his results the previous season. The car though, was very well sponsored, but most of this came from other sources.
Said driver was then offered an F1 testing contract on the basis that the F1 team boss assumed from the stickers on his F3 car that he could bring money into the team. The driver took the chance to get to F1 more quickly instead of taking a big money paid drive in Japan. The testing contract then came to nothing when the F1 team boss finally twigged that no money would be forthcoming, and by then, the drive in Japan had been snapped up by someone else.
I should add, that roughly contemporary to this, a full season in International F3000 in a reasonably competitive car would set you back around £350,000.