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.
I have heard of them, yeah. I thought feedzebirds just paid per tweet, not depending on your followers. Thanks though

no idea what I'm gonna do it for, but expect 2 for free when I start