The incidental use for demonstration purposes would not reduce the cost of the item unless you use it for like a year for demonstration purposes then the value of the item would be reduced by the depreciation of the asset.
So if you buy it for $100 and demonstrate it once then the cost of the giveaway is $100.
If the value (cost or you or depreciated cost) is $400 or more you need to give the "winner" a 1099-misc for the value of the item or a W-2G if the winner put money into the contest and they need to report it on their return.
