No frames are rendering, but the cost consumed is going up. Why?

In short, because that’s how render farms work. If the cost consumed is incrementing, then one or more parts of your job are currently rendering. Each frame of a range or tile of a still is rendering on a single node, with many frames or tiles rendering simultaneously. The rule of thumb is that you’ll have to wait until the amount of time passes that it would take for your machine to render one frame before any of the frames complete. This is how render farms work…many frames are rendering simultaneously, one per node. How many nodes, and hence how many frames, are working on your job is controlled by you via the Power slider.