Small enough to not get noticed, too little to cover their lifestyle for long, yet too convenient to not take :)
The big paydays usually happen through companies the politician and his ilk are in the board of, which just score very lucrative contracts or orders time after time. Or the politician is hired as a consultant for such companies, collecting fabulous kickbacks. Or the promise of early retirement into “window-looking jobs,” employment where they have a title, high income and zero responsibilities.