My tenant told me his front door key was stolen 3 weeks ago. My front door lock was a very expensive one, over 300 dollars, he did not replace it but instead added one of 30 dollars value. He moved out today. Is he responsible to pay for a door lock of the same value as the one i had?

I think that if you didn't have those details spelled out in your contract when he signed the lease then he would only be responsible for replacing the item, regardless of the value. Unless it is specified in the contract that any damage or loss to the rental property must be replaced with an equal or greater value item, then you have no grounds to stand on.