How profitable will my invention be?

If one of your first concerns is how much money your invention may potentially be worth, that certainly does not necessarily indicate that you are greedy.  It may instead simply indicate you are wise.  A patent's purest intent is to proclaim a new invention to the world.  Its most practical intent, however, is to provide right-of-way to the inventor to make money on it, by excluding others from copying it for a set period of time (20 yrs from the date the patent is filed, 17 from the date the patent is issued, whichever is longer).  So, from a practical standpoint, if you have no interest in profit you don't need a patent at all, and can publish your invention for the benefit of others anywhere.  Benjamen Franklin didn't patent any of his inventions (then again, he had already made himself very wealthy in the printing business before he ever turned to invention in his retirement at the age 42.)

Many factors affect the potential profitability of a patented invention:

Quality of the Invention
Naturally, for an invention to be likely to turn a tidy profit, it helps if it is good.  There are many examples that prove this is not an absolute requirement, but it certainly helps.

Timing
If there is a time and a place for all things under the sun, and that's certainly true of inventions.  Certainly you'd make much more on a saddle design that prevented saddle soreness in 1908 than in 2008 (but there still would be plenty of interested parties).  Every generation of inventions yields to the more advanced technology of the next.

Perseverence
Filing a good quality patent application clears the way for success, but follow-through is vital. Invention, being 90% perspiration according to Thomas Edison, is not for the easily defeated.

Marketing
An invention will not market itself, but via a license agreement, marketing an invention does not have to become a full-time occupation either.  Good communication and presentation skills of the inventor and/or any partners brought into the venture and right marketing strategy are essential...and as such, you should expect to, and will, find demonstrated successful techniques methodically outlined in The Essential Inventor's Guide.

Experience
Like anything making an invention successful is easier if you've done it before.  Of course, there's a first time for everything, and what the first time inventor can do is to learn from others who have gone before.  After all, what sense does it make to re-invent the wheel?  The Essential Inventor's Guide will mentor you through every step of making your invention a success, and equally importantly will teach you to avoid critical, but all too common mistakes that first-time inventors make.

Royalty
Naturally your profitability will scale directly by what royalty you are paid per unit of your invention product sold.  In The Essential Inventor's Guide you will learn what factors affect royalties and what you should expect to constitute a fair licensing offer for your invention.

Luck
As with any project, the final outcome will be influenced by factors beyond your control, and it is important to be realistic and accept this.  Because success of even the most compelling invention in never guaranteed, one of the cardinal rules you will find within The Essential Inventor's Guide is that you should never spend a lot of money on an invention.  (Naturally what constitutes a lot of money depends on one's personal circumstances, but the principal is universal.)  With The Essential Inventor's Guide you won't need to.

Beware of anyone or any organization that would have you believe that simply patenting your invention will automatically (or even probably) lead to success with no additional effort – they are usually trying to sell you something.  Patenting your invention is one step in a process.  That process is, however, well worth the endeavor, satisfying beyond most accomplishments, and sometimes quite lucrative.  It is the process covered from start-to-finish in The Essential Inventor's Guide, and it is a process that you can master.  Real invention is challenging, and real inventors do not shrink from a challenge.