I found the exact same thing with my mother. So far, she's just paying the premium (out of her Social Security check), but I went ahead and signed her up, as she is 86 and you know she's going to have some big time $$$ RX down the line.
Things did change a bit though, with a new prescription for an Alzheimer's drug for my mother - the cost saving under the plan for this one 90 day supply equaled her entire annual premium (Blue Cross Gold Plan, with no deductible, at $35 a month).
The signup process was horrible for me though - could only do it by phone and it took me between 6 and 8 hours to get thru and get it accomplished.