Is there any monthly payment or subscription fee?
Definitely not! In App Samurai, you don't pay for sign up. We don't charge our users unless they create their mobile campaigns. You only pay as you get installs (not clicks). You also don't pay for tracking your app's app store performance!
Is there any minimum budget to create a (boost/acquisition/video) campaign?
The minimum budget to create an ad campaign is $100 in App Samurai and you only pay for downloads, not clicks.
Is there a fixed budget I need to have to be successful?
The budget you want to allocate for campaigns changes as your goals, the market and location you want to target. There is not fixed price. The only thing you should remember is that the minimum budget to create an ad campaign is $100.