I've been using a dollar bill as a bookmark since it's convenient, I always have one, and it doesn't have enough value for me to fear it getting stolen.
Anyway, if your used a $5 bill as a bookmark for your kids. When they finish the book, they get to keep it. Obviously it increases in size as they get older to a $10 and $20 bill. Probably won't go to $50.
And I'd read the books too, so they can't pretend they read it.
I'm thinking we start small. When we read to them with Clifford or something, We keep a $1 bill in there and they can keep it, so the tradition can start young, like trading a tooth for a dollar.