I'm currently living in a very humid room and so am looking for a cheap way of getting rid of some of that humidity, without having to purchase an expensive dehumidifier. Are there any plants, objects, salts or whatever materials that can help achieving a more comfortable, less humid environment?
I think the comment about finding the source is very important - there is a reason it is humid, and unless this is solved, you will never solve the problem only band-aid it, which will cost more in the long run going against the "low-cost" part of this question.
The definition of low cost varies person-to-person. Under $100 is low cost to me, but maybe not to you. In any event, your best bet is a dehumidifier. A quick search on Amazon found some small used ones for under $50. You don't mention how large the area is so it's difficult to say if this will suffice, however it will most likely pull more water out faster than any plant or of silica packets will (I imagine you'd need thousands of these for even a small room).
You will likely require a humidifer that can be hooked up to a drain so you don't have to constantly empty it manually (hidden opportunity cost), which probably means a larger more expensive unit (still only a couple hundred dollars).
Of course you also need to consider electrical costs in this - at some point, it will be cheaper to properly solve the problem then to run the dehumidifier indefinitely.
Just open the windows! Most of the year the air outside will be less humid than the air inside.