Storyblaster only charges a 15% service fee on top of the amount you spend on your promotion.
When you create a promotion, the amount you enter in the promo window is the total amount that will go to Facebook. A 15% fee will then be added. For example, if you enter $50, that full amount goes directly to Facebook and Instagram, and an additional $7.50 will be added to your payment ($57.50).
As for a budget, we do require a $50 minimum spend, as we have found anything below that has limited impact.