In the expense rules you can create a rule based on the amount per night by choosing the condition 'amount' where you have the option 'per night (check-in/out-dates)'
To create a rule based on the amount per night, here are the steps.
1. Admin > Expense Rules
2. Click + Add expense rules.
3. Click Add Condition
4. Select from the list Amount
5. Fill in all the fields. Once done, click the Add rule.
To require the check-in and check out date. See below article.