Coco coir is a very good at holding moisture it is kinda like a sponge I can actually squeeze out water from saturated substrate. I am assuming you have a tank? I did this to solve the RH issues. I placed a plant saucer on the bottom of the cool side and then filled coco coir over and in in about 3-5 inches deep (cool side only) and I would about once a week poke a finger down and pour some water in to the saucer. It holds a large amount of moisture that slowly releases through the drier upper layers of substrate. I rarely misted and did not block the top at all.

Cheap yes it is, quite. Off the shelf at my regular home depot (in canada) it is 7$ for 3.3 cubic feet that is uncompressed That lasts me for 6 months for 11 snakes. I was getting larger blocks about 30 cubic feet for 18$ way cheaper but it was hard to store and hard to deal with generally and generally wasn't worth the savings....