24-pad, 0.5mm pitch: better to use QFN or BGA?
AFAIK, there's no significant cost benefit to using BGAs over QFN's. I think the tradeoff is that the BGA is roughly 1/2 the area; whereas the QFN can definitely be routed without microvias. If you don't already need microvias for some other part in your design, adding them to support this part might increase your blank board cost by 50%. 

On the other hand, are you sure you need microvias to route the BGA? The first two rows should be routeable without vias under 4/4 design rules. And the center ball has an escape at location b2, so you should be able to route that with a through via.