The original debt was the Crown's, having been run up by Robert & "Sons" (and Littlefinger, who was screwing everyone and putting money in his pocket for the last 20some years), with additional loans taken by Tywin before he died. They were totally willing to deal with Tywin, who's infamous for paying his debts, when he was running the show. They only made the loan to Stannis & called in the 10% of Lannister's debts when Tywin died, since they thought the remaining Lannisters didn't know how to manage money. Cersei didn't deal with the bank at all (directly) until she cuts a deal to pay off 100% of the debt in 2 weeks (not just the 10% that was being called in), and then actually delivers. They probably love that shit. The Lannisters probably have pretty good credit overall, especially as they're supposed to have all these great gold mines that produce a huge constant revenue stream. In like one generation, Tywin used the mines to go from being shit poor to being the richest dude. At this point, I think only Cersei knows that there's not actually any gold left in them. Order of magnitude on the debt Cersei paid off (Robert getting jerked around by Littlefinger 20 years + war of 5 kings) had to be way bigger than the cost of the golden company. We saw literal truckloads of gold coming in from highgarden. Iron bank probably really doesn't like the idea of having to deal with Daenyrs after the last time she bought something on screen... Where she traded her dragon for an army, then had the dragon burninate the dude and come back to her. Not great reputation if you want to do business. Tl;dr: ROI calculation on the Golden Company loan probably looks pretty good if the Iron Bank thinks there's even a 1% chance it'll keep Daenryrs off the throne, since the payout takes the shape of Danny's remaining lifetime of being able to make loans to the crown (without risk of getting lit on fire).