I don't know what's a reasonable rate per API call. But it costs reddit to provide this API for free, not only are these apps not serving reddit's ads but they are actively taking up server resources.
I’m the guy who built the server end of Apollo. I can tell you that there would be no problem paying for API usage. In fact, it would have been welcomed, as this is a sure fire way to support the service you depend on.
It's not charity. These apps are providing traffic, engagement and valuable content. Reddit should be paying them for bringing users to their platform.
