Like ste said.
Ship have different insurance price and the price augment as you add higher class gear.
So for say you have a sidewinder, by default the insurance is 1600cr for the stock ship, if you add new canon, shield etc the price will go up, mine ended up costing 3.2k cred.
You do not loose your money but if you dont have enough to repay your insurance, it'll draw on a loan box (dont know how to call it) and you'll get back with only 1k credits.
Think you'll have to repay the loan in some way, unsure on this.