We are increasing the price of promoting a post from 2,500 MLX per impression to 5,000 MLX per impression. The issue is that we have too much demand for promoting posts and we aren't able to service all the post views fast enough. As traffic increases and targeting improves we will be able to reconsider lowering the price.
We are also working on a feature that allows promoted posts to have an expiration date, after which time the unused funds are returned to the advertiser. This will reduce the size of the ad inventory we have to service and will make promoted posts more relevant.