I am just in the process of doing some major work on my girlfriend's GTV. Part of which was replacing a valve. The valve itself is only about £10, but it is the rest of the parts that you will need (or would be strongly advised) to change that are going to cost you.
I may be wrong, but if you are going to have the head off, then you really need to replace the head gasket when you put it all back together. The set is about £80. You would want to also change the Cam belt, balance belt, and probably ancillary belt. Approx. £20 each. Then you may need to change some tensioners and idlers, £20 - £30 each.
It really depends on how many miles your car has done.
I can strongly recommend ebspares based near Westbury for spares. Very competitve and helpful.