estimate the cost for cylinder head replacement

You need to clarify it. You are asking the cost of the cyl.head replacement, but you are saying the block is cracked. Anyway, if the engine was overheated it is possible that the head bolts pulled out the threads from the aluminum engine block while the head expended. If this is the case you have 2 choice: 1, replace the entire engine 2, find a good machine shop, who has the jig to install time-certs (repair the block) 9 out of 10 overheated BMW engine will lose the threads. Prepare for the worst. Zee