Bring the purchase receipts as documentation of price. Otherwise they can call up any page online that they want to demonstrate the price of the item.

On the customs form ( which you get on the plane), just state "gifts" or "presents". (You are not expected to do this in other than your own language, but anyway, you could use the Portuguese word "presentes".)

Personally, I'd just pick up my luggage and get into the "nothing to declare" line with the form. They may X-Ray the bags before you get in line though. Likelihood of a gringo being pulled aside to have luggage opened and inspected is probably small.

But then, although you may think you are doing your friend a big favor or being very generous, I and many regular visitors/travelers learned long ago that most anyone in Brazil requesting such expensive electronics is actually taking the 'bringer' for a patsy, a mark, a rich gringo, which is uncomfortable at the very least once realization sets in. My advice: Learn to say no. (Bring 'em a T-shirt or something from your hometown instead.) You may think this is cynical; I know from experience it is realistic.