You don't say where 'home' is.
The UK is probably one of the most expensive places to buy a DSO, trust me, I'm there!
Duty free shops generally sell cosmetics, alcohol and cigarettes with a few domestic electronic items. If your brother signs a declaration that it is for export, he might be able to buy one from a normal outlet without having to pay 20% tax. If he does that though, there is a small risk that the duty will be payable on entry to your country instead.
Brian.