please share your thoughts, could something like this even be profitable?
where would be the best place to advertise for this type of service to evaluate the level of interest?
No idea, But CC is a good place to start.
would more profits be made on the formulation, or manufacturing, or filling/packaging?
Depends where you want to focus and your equipment.
Sourcing: Why not leave that to the client? They deliver to you what is required. That is their headache. Not yours. And extra goes home with them.
Bottles: You act inadvertently as a representative for various companies, and have samples on hand. Client chooses bottle, you order from company.
Flat rate on setup. (based on # of ingredients/ types) Whether 100 bottles, or 5000. Your time and expertise is covered and per bottle/ tube the price is built in.
The greater the quantity, the more you make, and that is the time variable, but your initial set up is covered large or small.
You’ll have to run the numbers and include taxes, insurance and any other expenses, rent etc…electric…see if it is feasible and your breakeven point.