I didn't bid when I was offered Bula Bid (SIN-NAN-SIN). I think the lowest price was about 500 SGD. For this route, they didn't fill the seats. On NAN-SIN, I was pretty exhausted, and decided to ask at the counter about same-day upgrades. 500 USD or something. I didn't bite, and wound up sitting beside a stinky guy. Ugh!