Recently I came to know about the popular blockchain projects. What I don't understand is it costs much to provide the BC services (you pay to miners) but how are blockchain projects monetizing themselves!?
Even after some of them are open-source!
I work at BigchainDB GmbH (the company behind BigchainDB) but we haven't been very public about how we're generating revenues, or how we plan to do so in the future, so rather than get myself in trouble, I'll comment on one of the other ones you mentioned. BigchainDB may or may not use similar techniques. We have an Enterprise page on our website, which may give you some ideas of what we're doing.
IPFS is a protocol (or set of protocols). The main company behind it was initially backed by investors and then they had revenues from things like helping businesses understand IPFS or integrate IPFS into their specific applications. More recently, they were behind the Filecoin token sale, which sold tokens that will be used to buy and sell data storage in the Filecoin network. They have some revenues from that sale, and potentially from the future increase in value of any tokens they own. They might also sell commercial licenses for related software for a fee. I don't know. Maybe not, but it's certainly possible. Even if software is open source, businesses often want to buy a commercial license. Why? If the open source license has a share-alike clause, but the company wants to make software changes and not share them (under an open source license), then a commercial license may allow them to do that. A commercial license may also come with warranties or guarantees.