Our clothesline was already here when we bought the house, so I don't know how much it costs. However, I'd also estimate our savings at $20/month, and I feel good about the 200 kilowatt-hours worth of emissions that aren't being put into the atmosphere. Sunning really does help with stains, and I love the smell of laundry that has been dried on the line. Oh, it also helps keep our house cooler in the summer because the dryer puts out a lot of heat.
Our clothesline is one of the smaller, square ones. It only holds one load of laundry and is too small for blankets and sheets. However, my neighbor has a traditional, two-pole clothesline that she lets me use, and I love it. I use it to dry my bedding (which takes a long time in the dryer) and when I've run out of room on my own clothesline.