For a convention, "fixed costs" are costs which the convention bears independent of its size, typically because they must be committed to before its size is know, or because they come in fixed size lumps.
For a Worldcon, the convention center is almost always a fixed cost since contracts must be signed well in advance of the con and adding or dropping some space often doesn't affect the cost much. Insurance, the cost of running the Hugos and site selection, and technical equipment for major events are other largely fixed costs.
