Check who did the belt - the f4r engine is very sensitive to timing, and even Renault get it wrong, so you have to be very careful who you choose for belt changes. Alot of places will advertise a belt change, and will have literally changed the belt, without using any of the correct locking tools (if no locking tools used then the timing WILL be wrong) and it wont have been done properly.
A decent specialist will change the cambelt and tensioners and rollers, plus the auxiliary belt and tensioner (this is important as the aux tensioner can sieze up and throw the aux belt off - usually right into the cambelt, resulting in bent valves)
So this is why a belt change costs £450 + as it needs to be done properly.
Also, as Dan says, if the car has had crappy oil or has missed oil changes the dephaser could be knackered, which will add another £160+ to the bill.
So if the car is in good condition, will good history, redplaced consumables such as suspension and brake items and a documented full belt kit change from a renowned garage who has used the correct tools and know what they are doing, then yes it can be worth that easily.
But if not, you can factor in another £450 on a belt change (Or head to Dan @ SJM who has a special on with belts at £370)