Aerating your lawn helps to air down to the roots and it opens up space for the roots to stretch. It helps to establish a healthy root system.
A local lawn care company quoted us $60 for a 1/4 acre lot. We found that we could rent an aerator for $20-$30 overnight (5pm to 8am). We did three lawns one evening. That makes $10 per lawn.
We also discovered that "wetter is better". Although you don't want it to be soaked it helps to have a damp lawn...it helps the aerator to get a good deep core out of the lawn.
Turn on your sprinkler system before aerating and mark sprinkler heads with small flags. This will help you avoid hitting your sprinkler heads with the aerator. If you hit one of those little fellows your job gets more and more expensive.