[aelhaji]

Depends on what you would like to focus on. If you want to capture the complete demand curve for a product, then you're right. In that case the participants shouldn't be aware of any other alternative to get the product. But you could also choose to focus on users who aren't willing to pay the current price and better understand their distribution. This would allow you for example to determine which promotion would most likely turn a free user into a paying user.