Classic "how long is a piece of string?" question. If you want what they would cost to produce now, best wet finger estimate will be to get the original unit cost (NAO major project reports used to be a good source for recentish stuff - otherwise it's into Janes or similar - and apply escalation (defence inflation) percentage value since last contract date.
If it's current cost/value you want, the specific term used is "Net Book Value" - have a look through online Hansard and you should get there.
All of course with huge caveats as to what the UPC/NBV values actually represent! Can of worms and more....