It's the $9.99 book this month. https://www.logos.com/product/16036/institutes-of-the-christian-religion It's normally $40. (I think it's been higher than that in the past.)
You might want to check "This title is included in the following collections" further down on the page. It's part of several Starter libraries, probably available for a lot less than $10 for most users who own a base package.
