I booked the cheapest interior "guarantee" on my 3-night last month because it was the right price for a getaway, even going solo with single supplement. The Royal Up email will show the different categories with the minimum bid. I got a successful bid, and from that have a couple observations I can share with someone who hasn't done the Royal Up bid before.
First, the bid shown is per person (so if traveling solo it will be double that shown as a minimum) which was obvious. I had assumed though that the bid was also per night - which it was not. I didn't notice that the wording said per person for the sailing. Had I known it was for the whole sailing I might have included a couple more higher category rooms as part of my bids.
Second, since they use unfilled rooms to fill the successful bids there might be a reason why that room was unbooked. I was successful in getting a Promenade View interior as my upgrade, but learned the room was probably not booked because it was directly over the guitar player at the bar below. I'm up later anyway since I work a later shift so being able to hear what was being played didn't keep me up, but I could see someone who has an opposite sleep schedule not being as happy about the added sound in the cabin.
All in all, I'll probably bid again if the prices look good enough to me - though probably closer to the minimum because I'm already booking in a cabin I am willing to stay in for the price, and any upgrade would be a bonus to me.