Officials with Ameren Illinois say natural gas prices will likely be lower for their customers this winter.  The (Decatur) Herald & Review reports the average price for a therm of natural gas will drop between 3.5 and 7 percent this winter.  The utility has about 800,000 natural gas customers in central and southern Illinois.  Ameren officials say the price for each therm of gas will fall to about 53 to 55 cents.  That’s down from 57 cents last winter.  During the 2011-2012 winter heating season, central Illinois customers paid 62 cents a therm.  But just because the price is down doesn’t mean bills will be lower.  Customers who use more natural gas will face higher bills.