I disagree. I'd say the free month is not for a tweet, but for the number of users who sign up because of your tweet and then how much those users end up paying to use the product. If just one user signs up because of your tweet and pays to use the product for a month, then what's been given away has been earned back. And none of this says how much the product should be valued quantitatively, i.e. how much one month is worth.

On the other hand, you do comment about perception–if the users perceive the product to worth less because of this deal, "a tweet for a free month", the rationale behind why the deal is good for the product might not do anything to offset that loss in perceived value.

Your assumption, however, is that people wouldn't tweet about it if they had no incentive. The question now is: Does the tweet incentive help to increase the user base or not? That would actually be interesting data :)