That price is fine and fair.
Harman UK should be able to repair these and charge the time allowed in JBL repair costs PDF as available on the JBL pro site. If you had an account with harman you could get these with some sort of discount but perhaps they will be nice to you and not charge for the actual work, just the recone retail cost.
Anyway they should give you a total cost of the work before preceding VAT and all. That is not rocket science for them to do?
Then a authorized repairer on here can say if that is a accurate and fair quote. Doesn't sound expensive to me but JBL is the only speaker worth repairing to me