The credit rule of thumb
A credit costs what your membership costs divided by the credits it gives. A book bought with cash costs its list price less your member discount, or its sale price. Use the credit when cash would cost more; otherwise buy the book and keep the credit. The break-even list price is your credit cost divided by (1 − your discount): on the monthly plan with a 30% discount, about $21.36.
Prices are Audible’s US prices, checked against Audible’s own membership page. Outside the US, enter the prices you see.