Each item has a fixed starting price that is based on a share of the retail price, and a per day price for every extra day beyond four days. These shares changes by the type of item, the items retail price, and category (party/everyday).
For party items, the monthly price is usually double the weekly price, because these are items you won't normally wear for a longer period.
For everyday items, the weekly price is usually at around 70% of the monthly price, because these are items you want to wear more often, and for a longer period.
All rental prices cover the minimum logistic and cleaning costs associated with renting an item.