I remember picking up the hardback of 13th Age when it was first released. I was impressed, but like many of us, I have way too many RPG systems to run, let alone collect, all of the games I find interesting.
For 35 bucks for the 13th Age Humble Bundle, I can snag a pretty complete collection of the 13th Age in PDF, and that is damn tempting, as I'm am sure there is much within to borrow and steal for other D20 systems. The One Buck level isn't bad if you just want to check the system out (20 and 23ish don't seem worth it compared to the high and low extremes). Heck, there is even an SRD: https://www.13thagesrd.com/