It's £6 per month if you pay yearly. Also if you are a student you can get a 50% discount. Honestly it costs a negligible amount, that's less than a single day's worth of travel on the tube.
Ironically you could do the same with your replies.
And I'm so tired of the game of "describe a successful tech product in a way that makes it sound silly". Yeah, Twitter is just storing a load of VARCHAR(240)s, and Facebook is just an undirected graph, and Jackson Pollock's paintings are just splatters, etc. I could build it myself! Except... I don't, because it's not that easy, and I know it isn't when I think about it for more than 1.7 seconds.
Granted, it's really difficult to compete with free, and the note taking space is probably one of the most saturated software markets there is.