Online, buying in bulk and you're looking at around $2 each. At your local store, the cost could be around $4 after tax. Properly cared for, they can go for weeks without having to be replaced or rebuilt.
Is it really worth it? I found that it wasn't. My time is valuable. However, like luniz mentioned, it's a good skill to acquire. You never know when you either can't order new ones or make the trip to the brick and mortar joint to buy some. It's not terribly difficult, but expect to fail a few times as you learn how.
My first attempt was an unmitigated disaster. Second was almost as bad. Third, fourth and fifth time were like clockwork.
Yeah that's what I'm thinking... it would be nice to know but I really don't feel like committing the time to learning when they can be acquired for so cheap :/ lol