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You can put a portable humidifier in the room, one rated for 3/4 of the square footage in the entire house. I can specify a more detailed approach after I get to know more about the house and the instrument.
How old is the house, and is it brick or wood frame.
What humidity tolerances are specified.. 35 to 50%? or 50% plus or minus only 2 or 3%... that will have a huge effect on costs.
I also need to know what region of the country you are in, it is a lot easier to maintain humidity in Florida for instance than it is in Michigan in the winter.
We can go from there.