Is it necessary to have internal data always for benefiting from big data analytics or outside data can do the same job? what is the usual cost of seting up such technology in business?
Hard to answer such a broad questions. "Big Data" is such a catchall buzzword that it's kind of meaningless. Do you mean social media data, customer intelligence data, i.e., data that business capture about their customers such as purchase data, loyalty points, geolocation, facial recognition technologies, etc., etc. Are you talking edge analytics data that can be picked up by sensors through the IoT? Or are you talking about data created by systems calculating things like a customer's lifetime value, customer churn potentials, etc., etc.
When it comes to cost, you're also talking a huge range, there are a lot of social media listening systems that can grab social media data rather inexpensively, you're talking under $50/month in some cases, but in my work I deal with companies that have spend hundreds of thousands of dollars for analytical systems that would be considered "Big Data." Large corporations that are purchasing CRM/analytical/marketing systems that can track every one of their customers' purchases and then market to them when they might be on property are highly complex and very expensive and they require an impressive backbone of IT staff as well as analyst who can run the systems as well as create the models and send out the marketing campaigns.
Is it necessary to have internal data? No - you can do Big Data Analytics on data which you have sourced from elsewhere - however due to the *size* this is rare. Normally the data most valuable to you is your own. Can outside data do the same job? I don't understand - typically they will be data about different things, so no.
What is the cost? You can put together a Big Data Analytics system on a cloud for a few hundred dollars and a credit card - but it would only run for a few minutes. Or you could build one yourself for a few hundred thousand - or anything in between. It depends on what you need.
You need to be far more specific to get more helpful answers from us.