I don’t know about Hashnode, but let’s go through the whole picture.
You will definitely need some storage space that holds those comments. However, unless you allow image uploading to comments, this should not add too much to your budget.
This seems obvious, right? You need to add this feature to your code base. And, as it is the case with every piece of code, you will have to maintain that code.
And finally we arrived at the most important, and arguably most expensive part. Like it or not, you will face spam and harassment sooner or later. You either need an awesome community to fight this for free (in terms of money), or you have to pay some moderators who do it for you. Either way you will need a properly written Code of Conduct, and you should prepare to make changes to it often.
There are also some legal issues around here, but if you are creating an actual, open-to-everyone social network site you already have a lawyer to cover these, do you?