For the blog itself, I haven't used any money, but I did for instagram paid promotion.
So far I've paid something around $400 MXN (which is roughly $20 USD) in two months and thanks to it I've got around 15 registered followers more.
You can choose if you want your promotion to be to get more profile views, more messages or more visitors to your website (the one you put in your bio). I always choose for more visitors except for a $20 MXN promotion I made for the Ask Salamon event.
It really works better when targeting people from latin-america than when targeting english-speakers, and so far, I've got 100 or so visitors thanks to the promotions, but only 20 clicks to the actual links to the comic (whether it be via webtoons or tapas).
I'd recommend to give it a try with a quantity of money that you can afford, but linking directly to an episode you want people to read at the time.
Next time I will only promote when using a link to either tapas or webtoons, and leave the blog links for other instances.