Renting looks cheaper because you compare it against the new price. The honest comparison is against buying used and then reselling, which is what changes the answer for most books.
The actual formula
Your true cost of buying is the used price minus what you can sell it back for at the end of term. If that number is lower than the rental price, buying wins — and you keep the option of holding onto the book. Rental fees are gone regardless.
Rent vs buy calculator
Buyback offers are typically strongest for current editions in good condition, and drop sharply once a new edition is announced.
Rules of thumb
If the used price is within roughly $15 of the rental price, buying almost always wins once resale is counted. Rent when the book is expensive, the edition is about to be replaced, or the course is outside your major and you will never open it again. Buy when the book is in your field, when you might want it for a later course, or when the used price is close to the rental fee.
The hidden costs of renting
Rentals carry late fees, damage charges, and return shipping deadlines that land during finals week when you are least likely to be thinking about them. Highlighting and margin notes — how many students actually study — can trigger damage fees. Read the condition policy before you rent a book you intend to write in.
When neither is right
Check whether your library holds a copy on course reserve, whether an open-access edition exists, and whether the previous edition is acceptable. A free copy beats every price comparison.