I just won my Royal Up on Freedom from a guarantee balcony (it had been assigned to me already as the basic balcony cabin) to a spacious balcony. Minimum bid was $45/person, I bid $75/person. I had been checking inventory right before the 45-day check-in mark and noticed quite a few of the spacious balconies and junior suites were still available, so I bid on both knowing that. I bid as soon as I got the RU email, and literally the next day I couldn't adjust it anymore because it said "pending" (the JS one said "expired"). Within 24 hours, RU confirmed my upgrade.
As someone else said, I think it depends on inventory available as well as how popular the room you have at the moment will be for resale. I also looked at my budget and said I'm willing to spend X to do this but not more than that, and that was it. Good luck, because it's truly just luck with RU!