Even if AaronFriel's assertion is true and they received no special deal, it's a naïve view of corporate governance and SAAS v.s. in-house devleopment to say that (as an example) if their license fee was high enough that they'd start incurring a net loss in 10-15 years that they made the wrong move.
For Citibank that was US$500 million then, which they could either invest directly or return to investors. The opportunity cost of that cash has to be factored in.
It's also an a large ongoing liability to have a sizable in-house development effort when it's not your main business. Departments need to be staffed and run, business plans made etc.
For all of Oracle's flaws they're presumably going to have an incentive to improve the software, and more capital with which to do so from clients other than Citibank. For obvious reasons other banks would be more inclined to buy from Oracle than a direct competitor.
There's also often tax reasons for why it's preferable to buy a service v.s. maintain in-house software, and naïve back of the envelope math usually doesn't account for that.