Try your local cleaners

Most of them do hems and other small alterations pretty cheaply.

If your pants are expensive, you might want to have someone really good do them. I use the Dress Doctor in Water Tower for alterations on "good" clothes. She does a fantastic job but would not be cheap (e.g I'm sure she'd charge you a lot more than a cleaners). If you're interested call her for a price (google her website for contact info).