1998 Dodge Caravan

I have dodge Caravan 1998, it has 97,000 miles and I was out of town for a month. When I came back, the car bettary was dead. I replaced the bettary. But I got the new problem, when I start the Van, the lights turn on automatacally and never turns off, and even if I turn off the engine, but the headlights stays on all the time and it drain the bettary. I took it to the mechanic, and he told me the problem is with the Body Control module.
Now my question is, if it is true then how much it cost to get it fixed or can I fix the problem and how I can do it?

Thanks.