Renault Megane Head Gasket

Hi, can anyone help me. I have a Renault Megane 1.4 RT and have been told either the Head Gasket has gone or the Head is cracked all the coolant has gone into the engine or something like that either way it won't move. Can anyone tell me how much either of these should cost to have done in a garage not a renault dealer. I have been told if it is the Head Cracked I will probably have to get a second hand engine put in. I have been told it will cost about £800 if it is just the gasket or £1000 if the head has gone and I need a different engine. Just thought it sounded expensive. Thanks