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!