Considering you have a crapload of topic tests and a very mean solutions (by that i mean so tank that i would honestly jump on it just to make babies), $40 to me is very cheap.
I only own Checkpoints, A+ notes and Exambusters for my Maths Methods when i did my Year 12 (this year), and i'd like to say that I didn't even touch Exambusters as it didn't even appeal to me... yeah it did show the tricks and stuff, but i could have just picked that up with my A+ notes. what a waste of money in my opinion.
If you are intending on making trial exams, don't make it over the top like Kilbaha 2010, cause that was a real mind fudge. Maybe you could have like a topic test that is in the intermediates level, but have an advanced right after it for students who really kill the exams. Or maybe even release two seperate books, one for people who are just aiming for 30, or people who want to aim very high!
The only problem however is you'll need someone to check over the solutions (cause companies tend to make errors). Derrick's book also had errors, and you don't want to distribute a book full of Errors (Checkpoints Methods 2010).
Nonetheless, good idea =)