I think it is the support cost of 3,000 soldiers in a province that has just rebelled that bankrupts the rebels.

In the VI campaign, I've seen the rebels of Strathclyde build up their province and churn out more soldiers.

When I want to induce a few rebellions to give my troops some practice, I give the governorship of Scotland or Portugal to a general with some happiness reducing vices. Assassinator is best because any province he governs suffers a 40% drop in loyalty.