If that is your definition, then the answer is "everything". If your definition is "lots of money" then the answer is, as always, very few things which are a lot of hard work.
If that is your definition, then the answer is "everything". If your definition is "lots of money" then the answer is, as always, very few things which are a lot of hard work.
Okay. An example: Spend 10 years building knowledge and reputation on some field and become really good at it. Then give a one monthly paid talk at some conferences. That goes $5K/talk. Pretty much achievable. You can even make more, depending on the experience and value you bring.
Step 2: Solve problem in a high-quality, maintainable, relatively bug-free way
Step 3: Charge a lot (so you don't need as many customers)
Except to add new features, you can literally watch the money roll in since you've somewhat automated getting the customers. And don't do stupid things like iPhone apps.