I believe I understand how to calculate the cost in broad terms. Objects seem to have a fixed cost of 10, while fields have a cost of 1 (times any multiplier on the object).
I was looking for something more exact/specific. For example, Boards has an items_count field, which apparently “comes with a high complexity cost”. The cost for this specific field seems to be ‘5000’ (from testing in the playground). Is there a list of these “edge cases”?