At one thousand images per month, you are paying roughly twenty dollars. This is comparable to what remove.bg charges at the same volume, and slightly more than the free tiers of some alternatives.
At ten thousand images, the cost is around two hundred dollars. At this point, running an open-source model yourself on a rented GPU becomes worth considering. A single mid-range GPU instance can process this volume in a few hours per month for a flat hosting cost.
At a hundred thousand images, the calculation changes further. Cloud GPU pricing means you can process this volume for under a hundred dollars in compute if you optimize the pipeline. The API convenience premium gets large.