Slave should be about £100 ish for a slave cyl. Your quoted price doesn't sound too bad. It is a pain of a job, I think its a long difficult job. Big chunks of gearbox, ird etc to move. I would say at about £250 parts and £300 labour then for the £300 including a guarantee (if the garage supply parts) its worth getting the garage to do it. Alternatively if you have lots of time, decent tools, a good jack and axle stands then give it a go.
Good Luck.