As is widely known, I work for an insurance company resolving claims; in this capacity I have often encountered 'small tips' costing in excess of €2000. A lot depends on the type/age of vehicle that you hit and the type of vehicle that you own. For instance VW's are very solid vehicles and can easily crumple the boot floor of a lighter weight vehicle.
Your insurance company would have employed an assessor to negotiate with the repairing garage. They would have done everything possible to reduce the cost of the repair - as there's no way that they will end up recouping the total amount from loading your premium.
As to Santanta saying that you could have got another mechanic to look over the vehicle - it may be possible sometimes, but usually the damaged party is entitled to take their vehicle where they like and unfortunately you/yr insurance co. have to wear it!!
Yr insurance company should be able to give you a complete breakdown of how €2600 was reached without any hassles - can't understand why they're being difficult with you. Its possible that the repair was only part of the pay-out - depreciation, inconvenience, car-hire for repair time and loss of earnings for the day of the accident would easily push the price up to €2600.