I have owned, been president of, grown and sold four companies: a volleyball company called Spike Nashbar, a bicycle company called Bike Nashbar, a computer software company called Nashbar\Associates, Inc. and a school uniform company called Choice Uniforms. As a part of running those businesses: I have managed as many as 350 people, grew sales to over $35 million per year, received a patent and sold those companies for as much as multiple 8 figures.
Through my membership with the Young President's Organization, I attended executive education at Harvard University. I am also an instrument-rated (though not current) pilot.
I now run FaxBetter, an internet telecom provider that has serviced over 500,000 customers to date and have invested seed capital in a patent-pending digital marketing technology.
When the cost to acquire each additional paid user:
-is less than the prior user you are in viral growth.
-is equal to the prior user you are growing.
-is more that the prior user than you are starting to reach maturity (but possibly not there yet).
-is equal to what you are willing to pay then you are at maturity.
-is greater than what you are willing to pay then you are in decline.
Define your potential customer segment and ask them (Would they purchase your product at $X? How many/often would you purchase? Etc.). You can do a statistically significant, +/- 4+%, marketing research survey for about $500 at SurveyMonkey.com.
The survey results should help to answer your other questions.
Cheers and good luck!