Cell tower agreement

by bob
( Illinois, USA)

I have two cell towers located on my property the leases expire in 2013. The owner of the two towers has proposed that I lease an additional 1200sf of space provided that I sign an agreement extending the lease on the two towers until 2043. Located in suburban Chicago I receive about $1000.00 per month for each tower. The owner of the towers has proposed that I accept the current rent and a 4% percent or a number less than that based on a price index. I have talked with an attorney who agreed to negotiate the contract for 20% any increase that is obtained. To me that seems like a lot is this the going rate for this type of service?