I have a central humidifier in my basement. There is a drain on floor, connected to humidifier, and I guess it was clogged somewhere, water was flowing everywhere. I turned it off, and the water stopped.
Should I call a plumber or is pouring liquid unclogger going to work fine?
If I call a plumber, how much is the reasonable cost to fix? I made some calls and prices ranged from $170~350.

Thanks in advance for your inputs! :)

If the water is backing up out of the floor drain and you can get the humidifier drain pipe out of the floor drain you can then use a plunger to plunge the floor drain, this will sometimes unstop the drain, after you do this try pouring water in the drain to see if it is open. If it is not open you might try a water hose in the floor drain with rags stuffed around the hose to make it water tight, now turn the water on gradually and hold the hose in the drain, you will feel a build up of pressure. If after a minute of so you feel the pressure go away that is a sign that the stoppage is gone.

Those are some fairly high prices.

Cost of drain cleaning depends on where you are located. It is a simple job and as suggested by Letmetellu, you can do it yourself. If the cloggage won't let go, call professional.