Cracked Cylinder Head...

I have a 1993 Vauxhall Cavalier 1.7L turbo diesel (with the isuzu
engine) and I'm having some cylinder head problems...

I started to get symtoms of a head gasket failure (white smoke from
exhaust, loosing coolant, white scum on the oil cap, etc.)
Unfortunately I didn't get around to doing anything about it until
about 500 miles later, but the coolant never ran out and to my
knowledge the engine did not overheat.

The mechanic had a look at it, said that the head gasket had indeed
blown and replaced it. About 500 miles later, the same problems
started again. The mechanic assumes it must be a crack in the cylinder
head (he didn't get a compression test done on the head when it was
skimmed).

My questions are:
1. What could have caused the cylinder head to crack (I thought it was
something that only happened when the engine overheated?)
2. If the head is sent away for repair (welding I assume), what is the
likihood of this repair lasting for any duration?
3. How much does a new (or good condition, second-hand) cylinder head
cost?

Is it worth getting this repaired, or should I give in and buy a new
car?

Any advice appreciated -- I'm not very mechanically minded!
Thanks,
Ash.