the gasket is something in the order of 75 euro's
depending on the state of the head, it needs to be polished expect
another 75 euro's.
since your there already, replace the cambelt as wel, that's another
75 euro's + the labour that should last no more than 6 hours,  
i'd say it sets you back the best part of 700 euro's

Have a look at the corner of the head where it joins the block under
the thermostat housing. Can you see a crust of dried antifreeze? This
is a well known fault on 1.6 twin cam engines, and there is a school
of thought that says it isn't the head gasket but coming from the
thermstat housing to head joint, which is RTV sealant. Could be a
cheaper option to get this resealed first.

I just had to replace the head gasket on my 02 307 HDi (110) for the same
reason and that cost £1113.51 (strange that number seems burned into my
head) at a local dealer.

You're lucky I was losing 1.5L per day until I took it to a different garage
and they worked out what was wrong :-(

Stephen