Hello:
Thank you for info.
I have contact with my Esri local reseller.
It seem that if you have an arcgis server license, using flex viewer is free.
If you haven't then you need to pay a 6.000 $ annual fee in order to use it.
Respect to cloud base services, 10.000 $ are too expensive for us. We are a small company and we need very small hosting capabilities.
Arcgis for subscriptions could be a good alternative, but it will depend on prices plans.