Here in Houston, the problem is usually too much humidity, but it does occasionally get very dry here -- like down into the 20s. This past week we've had unusually mild weather -- temps still a bit high, but quite dry, with the RH dipping down into the high 30s. I don't worry too much about humidifying my shop at that level. But if it gets much lower than that I will plug in the humidifier.
I use a smallish bedside-style humidfier. Cost me about 12 bucks at Wal-Mart. Capacity is probably around 3 liters and it'll run about 10 hours or so before going dry. I set it on the floor of my shop and let it run. My shop is about 12 square meters, and I find that this little humidifier works well enough to bring the humidity up into the 40s. If it gets really dry where you live, a couple of these will probably do the job.
To de-humidify the shop, I use a window A/C unit during the summer, which will not only keep the shop temp comfortable, but gets the humidity down into the 40s pretty consistently. I also have a dehumidifier that I'll turn on after I shut down things in the shop. During the cooler weeks of the year (from mid January to mid February or so
) I'll run just the dehumidifier. Unfortunately, neither a decent dehumidifier or a window unit A/C are particularly cheap (plus they also run up the electric bill more than I'd like), but I went ahead and shelled out the bucks for them because I knew they would be needed.
Best,
Michael