Few hundred a month, i will need to check to get you exact numbers, I have posted them in the past though.
But if you run a smaller instance (such as a single person), or happen to not keep the enormous history QOTO has (which we keep for searchability) it will be much cheaper. But cheaper comes with a cost, the degree of federation of a server is determined by the cumulative number of mutual followers across all its users. So smaller instances, while cheaper, tend to have a much smaller view of the fediverse and are seen by far fewer users (mostly in terms of federated timelines but also in terms of what comes up when you search for a hashtag and other things).