Regarding your prices, I find them too low. I only have 1000 followers and still get paid more on FeedZeBirds (see below). You could easily set a price between 0.35-0.60BTC/tweet and sell like crazy, especially since the tweet does not get deleted.

There is FeedZeBirds for Twitter advertising with Bitcoins, by the way. Great service, immediate payouts, easy to use.