I went to get it and it was gone when I tried so I'd love to find it for under $20 as well. I need it now that I ordered Among Thieves for $38 thanks to it being in my Gold Box.
Just rent it. I rented it 2 days ago, started playing around 10pm and beat it by 5:30am with over half of the treasures found and on Hard difficulty. Another few hours would have me 100% trophies if I cared about them. Its a good game but short as and lacks replay value like most single player only games.