If you look at all the recent flaming wrecks of Very Light Jet companies, they all significantly underestimated the time and cost of certification. One of my colleagues has stated that if you want a VLJ certified, you'd better plan on having close to $1 billion available.
And if it's a new type of aircraft, like a tip jet, plan on lots of issues with any certification authority because it's not the 'normal' technology.
And unfortunately, most of the folks who start these sort of enterprises don't understand the complexity of getting something certified. The folks who work at big companies, who should know, often seriously underestimate the problem.