The biggest cost for a bike race that touches public roads (aside from prize money) is paying the locality for closing down the roads. That entails police security which is usually how the locality gets compensated (they give you the roads but require police, then charge you for the police - some cities are fairer than others on rates). The promoter also pays for officials and scoring. The rest is usually volunteer labor. So, you are correct that once the course is set up, filling your allotted time with more races does not cost much assuming you are not expanding the time you are already budgeting for.
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