To actually make money you will need to be a Certified Developer and sell in the Content Store. Everyone can sell at the Market Place, but you will be paid in DA points, which you can then spend in the Market Place. The exception again is that when you are a Certified Developer you can cash out points.
I am not a certified Developer, so what I'm telling you is from memory. I have a few things in the Market Place, but they are free.
Before spending lots of money, I would start by looking what is currently available and what you can offer that is different.
I personally don't believe doing this is very profitable, unless you are a well-established developer that has been around for a long time.
EDIT: Fixed typo...
Edited
6 Years Ago by
animagic