I have always gotten 2-3 hours pretty much no matter what I do.
Download CoconutBattery. It'll let you know all about it.
If the current battery capacity is low, then calibrate it. See
here for that and more.
My current battery is about 18 months old. It was down to about 60% of original. Calibrated it once a week for three weeks and was able to bring it back up to 91% of original capacity. After you do that, calibrate it once a month or so. Other than that, use your notebook like you want to. (I don't follow my own advice, I just do what I want with it all the time.)
While $100 sounds like a lot for a battery, for what they do vs the number of months they will last is really an insignificant amount. At only 20 months you're down to $5 a month expense.