Is there such a thing as biblical bribing?

We have told each of our children that we will pay them to memorize entire books of the Bible. Like you're probably thinking, they started looking at some of the shortest ones first, namely 2 and 3 John. We have committed to paying them $20 for books up to four chapters, $50 for books up to nine chapters, and $100 for ten or more chapters. Are we crazy, misguided or brilliant for suggesting this?