Treat her to a nice, dressy dinner with a friend or her boyfriend. Treat her like an adult. Then ditch your mom (or not, depending on your family) and take her to a club that is 21+ or a music event that is 21+ (house of blues tends to have these) or a bar that only lets folks of age in because anyone else is a waste.
The nice dinner will probably run you $30 – 50 a head. The high end is if you include fancy alcohol. If you can, find a nice place near you that lets you BYOB. Then get a decent bottle of wine or six pack to cut down on costs. The club, I have no idea as that cost varies wildly depending on where you live. In VA, the only club I went to charged $5 cover and was very reasonable about drink prices. I’m pretty sure in Chicago that is less true.
If you do take her to dinner, find a place you want to go. Look at the menu online. Decide if the price is right for you. If not, look at another place and their menu online.