Beyond economic repair

How do you decide when something is beyond economic repair? A while back I bought a rather nice lens second-hand for £400; new it would cost £690 (maybe a bit less depending how much time I spent shopping around). Since then the AF motor has failed and I've now put it in for an estimate to repair it.

My current thinking is that £200 seems a sensible limit (being half what I actually paid for it and still keeping my total expenditure less than having bought it brand new in the first place). Any other views?