Few years ago I wrote a solution manual to one of my text books (half of it was part of the assignments). I sell it on Amazon for $30 for pdf download. Been making around $200/month.
How much time did you put into this upfront? Seems like the most "passive" item in the entire thread so far. Can I ask what book this is for? whats the lifespan of a solutions manual like this? Have you been selling it for several years or does it obsolete itself in sync with the aggressive textbook versioning schedules?
I guess I am lucky that the text book still gets prescribe in some universities. I also think there is no other version of the text (I need to check that) I do not maintain it at all. And like I said, I solved most of the questions as part of the assignments and later worked out the left over ones, packaged it into a pdf.
When my professors found out that there was a solution manual floating about, they changed the textbook. Apparently all of the student's answers started looking the same.