There are two buttons when choosing to proceed with a book if you have a Kindle Unlimited subscription. The TOP button says something like Borrow Now For Free with Kindle Unlimited - and it does what it says on the tin - allows you to borrow the book for free. The BOTTOM button says Buy Now - and, you guessed it, allows you to buy the book. Does that cure your problem? Alternately, are you looking at an old advert for a book saying it was free hours, days, weeks or months ago. In which case the price offer is probably no longer valid.