I like to get my hands on the cheapest edition I can in the best condition at that price. If the words in the book are the same as a more expensive one, I’m getting a better value, in my opinion. Then if it’s a story that becomes really important to me I might consider upgrading to a “nicer” or collectors version, and keep the old ones as lender books.
I’m happy to pay a bit more if the book contains colour illustrations, or has a particularly gorgeous binding. However my maximum for a single book is £35. Unless it’s a textbook that I’m told I have absolutely no choice but to purchase and second hand copies are unavailable.