People who have run an online conference, how much does it cost?

I've seen several conferences go this route recently, and both costs and the fees charged have varied wildly, but for good reasons.

The key components driving cost are, in decreasing order:

• Salaried staff (especially if the conference is a major support for a society or similar organization): tens or hundreds of thousands of dollars
• Publication fees (e.g., for archival proceedings of an IEEE conference): thousands of dollars
• Organizational costs (e.g., servers, corporate fees): thousands of dollars
• Teleconferencing software: hundreds of dollars

The conference fees thus shift accordingly. The models that I have seen so far are:

• The large conference supported by full-time employees didn't change its fees much at all.
• The mid-sized conference associated with IEEE charged authors about half the normal price (since their papers were the dominant remaining cost), and everybody else a pittance.
• The mid-sized independent conference charged PI and industry attendants a couple hundred dollars to cover their fixed costs, while students were free.
• The small interest group meeting made everything free.

If (as it sounds) you are running a conference for just a few hundred people using all volunteer labor and don't have fixed costs, then something like a monthly Zoom (or equivalent) webinar subscription would likely be sufficient to manage all the online aspects (assuming you let connectivity be the problem of the attendees). That will cost only a few hundred dollars, and you can likely cover it with a low registration fee --- or maybe even just use an institutional account from one of the organizers.

All four I know of have cost zero. However, they have piggybacked off university licenses that the host institution already purchased for teaching classes and some small amount of university IT support, and various organizers may also have chipped in small amounts of personal funds for small items.