$1, No, wait, $1000 maybe even $100,000 up to $1,000,000. I am sorry, but it is all location location location. An acre near Chesterton could be $10,000 an acre while around Valpo could be $15,000. Your question is hard to answer cause it all depends on where it is. An acre on 30 could be worth more than an acre on 450N!
I would suggest to contact a local agent and tell them what you are looking for and they will be more than glad to help point you in the right area. Or, if you are looking for what someone sold an acre for go to the court house it is public record most of the time.