Which manufacturing method most economical?

The size of your product will have a large bearing on the cost of moulds and the pieces required will really determine which method is the most economical for TOTAL cost including mould amortisation. The pieces required will also bear on mould costs eg aluminium is cheaper to produce but not suitable for long production runs (need steel). All said though fibreglass moulds will generally be significantly cheaper than injection moulding.

I will agree with the first commentator. Fiberglass layup and finishing is very labor intensive. Even if your injection mold costs $100k it doesn't take very many thousands of parts made manually to over shoot that cost. The injection will also be much faster. You might also consider compression or transfer molding. I really need to have more info and preferably see a part drawing to make a serious recomendation. Please, if you are not sure about the technology get some consultation. This is something I can do for you and from what I have read on this forum there are others as well who could help. It is very easy to spend many thousands of dollars and end up with a total disaster if all communications are not clear when you purchase tooling. If you like, I have a presentation I used for a talk at a local inventor's forum that I could email to you about manufacturing consulation.

A hint to the size of the item you wish to make would go a long way toward helping others offer helpful advice.

Also what is your definition of "mass production"?

Hatteras Yachts considers 30 yachts a year "mass production."

Hasbro Toy would consider 100 items a minute "mass production."

A candy company would consider 1,000,000 pieces an hour "mass production".

Half a question will only get you half answers.

You mite want to think about vacuum forming.