The cheapest business jet type rating I've seen was about €12,000 for the old-generation Cessna Citation (aircraft type C525), offered by a pretty reputable Central European ATO. But there are two substantial factors making it as reasonably priced as it was. First, abundant supply. It's not one school in the world holding the monopoly on training for this type. There are multiple ATOs in different parts of the world which offer it. Second, this jet is not exactly top-notch. It's small, relatively old and far less impressive than the average "pocket rocket". It's a great workhorse of private aviation, but it won't make you stand out in the way a Gulfstream or a Global will. Hence - reasonable instead of "premium" pricing for the rating.