While the Citroen ZX is an up and coming modern classic, it's still got a way to go yet. This generation of Citroens take a while to get there - just look at BX. Prices were in the doldrums for years but now good examples are fetching strong money. The short answer to your question is 'no' - fixing a head gasket on the ZX is probably going to cost the same as the value of the car - especially as yours is a fairly ordinary model (prices for which are always the last to take off, sadly). But, if the car is in good condition and has plenty of history, good bodywork, low miles etc, then you could regard it as investment and wait it out for prices to rise.