What is the estimated cost of incorporating a Billing and Online Payment function on a mobile app/website? And what system/option are there with good cyber security?
Not sure I understand the question right!. Are you asking about mobile and online payment gateway to use in your mobile applications ? if so then the best are Strip and Square. The both have nearly the same fees but you can find more at
Implemeting Stripe with a freelace dev can cost you up to 10hs depends on your software infrastucture. 10hs may cost between 100 and 700eur depends on the hourly rate of the developer.