How much are you willing to pay for a used book? Personallly I feel like I'm getting cheated for a novel if it's more than $10 AUD ($7 USD), save for anything particularly niche/rare.

I have spent over £100 on a used book 😎I think for rarer copies (and I don’t mean first edition LOTR or whatever) it’s the worth the value. Websites like Internet Archive and Anna’s Archive won’t last forever and once they’re gone so many books will become impossible to find.
It's also just such a madhouse in Aussie secondhand stores. I can buy 12 Joseph Conrad books for a combined 15 dollars and then the next store over will ask for 10 bucks for their chewed-up and sunburnt paperback of The Tin Drum. There is no rhyme to any of this!
Depends. Vintage books or thicc coffee table books I've gone up to $40, for paperbacks and hardcovers I max out at $4.