In the late 90's I took an intro psyc course and the prof wrote his own online textbook, mass 300 student lecture and we all had to "subscribe" to his book for $40 each, he taught 3 sections a semester. He had $72k a year on sales, because you couldn't pass the course without the "book". It was one the most god awful eye bleeding design I've ever seen. But, yes you CAN make decent money off selling your own textbook to students on the 101 level.