I'm really not sure if you voided a warranty or not by modifying it,I doubt it.But one things for sure,if you void a warranty of any item,you simply have to pay more,especially if its oem.
Expect to pay a little more than half of what the phone cost new.
Yea,they will sell you phone insurance,as it mostly covers internal damage(Broken screen,chips,stolen,etc.) I'll still look into it to make sure,they change that agreement page a lot.