FCP 7 might be in demand now that it has been discontinued. It was very liked software. Price is based on demand. If they offer $500 (sorry, I'm in the US, and I have no clue where the Euro or Pound symbols are), and no one buys it, they might offer less. Or, if they offer $800 and people buy it...well, that's what they'll sell it for. If it's new, they might be able to ask full retail, as people will be willing to buy it. Used, you might get it for less.
Bargain. Say "Hey, this software is 4 years old, and discontinued, I can give you $200 for it." If they want more...bargain until you find middle ground. If they won't, bargain with someone else.
Price is always based on demand though. If they think they can get full retail, and someone is up a creek because their post facility is all FCP based, and they need to add 4 edit stations...they'll pay.