If you do not have one. Get an I-Pass. It is good anywhere E-ZPass is accepted. You do not have to stop at toll booths and the toll is cheaper.
You give them a credit card number and have them charge $40 to it. Tolls are deducted from the $40 till the balance is $10. Then they automatically bring your balance back up to $40. You have one transponder. You tell them each vehicle and its plate number. They figure out the charge as you pass through.
I am going too Elkhart in May from Madison. I know of one toll for $4.65 on I39. I80 in Indiana is going to cost $27.28 for four axles. My route will be I39 south to I80. Then I80 to Elkhart.