No monthly fee, or any other fee for that matter, on the Illinois I-Pass. It is compatible with EZ Pass for cars only.
From the I-Pass web site:
Vehicles (including their trailers) that exceed four tires and two axles, and that are traveling on E-ZPass roads, must exchange their I-PASS transponder for the compatible E-ZPass "blue" transponder available at every Tollway Customer Service Center and at select Road Ranger outlets.
My I-Pass handles our cars or the truck and trailer on IL tollways. When we head for the Northeast next summer we will exchange it for the "blue" transponder.