Technically, I paid nothing for Dropbox - as to get the app is free (plus, you get 2GB of storage free).
However, I am paying $9.99 (USD) monthly for 50GB storage (actually, $99 a year because of a "discount").
So, in a way, the price of the app is un-measurable, as I intend to keep Dropbox for a long time, and by paying yearly, I keep adding to its cost.
However, $99/year is priceless to me to have "in the cloud" storage for (1) access from any computer and (2) a back-up system. YMMV.
Marilyn
[Who's not sure she got her idea across ... but hopes people will understand what she's trying to say...]