Transponder faulty in key
MERCEDES-BENZ M-Class (W163)

My ML 320 1998 Mercedes car battery died. I got a jump went home. In the morning battery was dead. So I got a new battery and put it in the car. Tried to start it. It turned over and died 2 seconds later. Replaced fuel pump. Car still turned over and died 2 seconds later. Got it diagnosed. It said Transponder faulty. Car doesn’t recognize the key anymore. I have used the same key for 2 years and it’s just a key. I have no fob. One guy says new key programmed. Another says new ignition. Locksmith wants $450 for new programmed key. What can I do. I’m 72 years old on a small pension.  . Can you tell me which way to go that is best to fix my car? . It has 283,000 miles on it.