One of the approaches my statistics books take is to include the numerical answers in the back of the book but have a separate guide for sale if you want to know why specific approaches were taken. By being an extra book, students who need the extra help pay extra for the time to compile all the extra solutions. This also keeps the cost low for people who only need a reference.
When you say 'my', I can't tell if you mean books you've written, or books you've read. Either way, writing the solutions is non-recurring work; the cost to print the extra pages should be pretty marginal compared to the presumably > $100 cost of the textbook. To my mind it looks more like 'we get two bites at the apple', rather than saving the poor students the extra $10 to get the solutions with their textbook. Of course, I don't know how much the solutions manual costs, but I feel like this approach compromises the student's experience, when they should really just get all the content up-front.