We are migrating to Booksleeve, which was developed by the people who have built StackOverflow itself. So far we had very good experiences.
The money is not the issue (the cost of migration is higher than paying for the license), we’re doing it because we just don’t want to enter a business relationship with company with sketchy practices like this.
(And no, we didn’t accept any license or anything, all we did was a git pull from GitHub.)