20 valve turbo coolant loss

[McCohens at aol.com]

I am seeing an empty burp tank every morning on my wife's 91 200Q.  No leaks, no white smoke out the exhaust.  A pressure test showed a small drop over time but nothing that terrible.

My mechanic is hesitant to take the car in as he has seen cracked heads on these in the past and has had problems finding good ones as a replacement. 

Any other experience in this area?  Never saw this sort of issue on the list before.  If my mechanic is correct what is the going rate for a good head.