Hi goncalo, I don't think buying a hygrometer is too super important. I've owned several ranging from the dial-shaped exoterra type to more expensive electronic ones and so far not a single one has held up well long term in the high humidity of a dart frog tank. The readings aren't very accurate to begin with either. Without paying a lot of money for an expensive one, I don't think you're going to be able to find anything that will be very accurate.
So, what I do is I just estimate. If the glass fogs up a bit every so often, then there's probably enough humidity. Eventually you get a feel for it. If you're not sure of yourself when you're starting, then by all means go grab a cheap hygrometer and use that to spot check your guess. Learn what "good" humidity looks like in your viv. Just bear in mind that it's reading won't necessarily be accurate either.
Yes, that and you've got most of the top sealed off. It makes it so that the water/humidity doesn't get out very easily. But yes, if it's well planted and set-up then humidity isn't much of a problem for most people. Exactly right.