Your monthly spend numbers are used to give potential buyers an indication of your site's profitability so should include every outlay your site incurs to operate - both fixed and variable costs.
This should include:
- Infrastructure costs: hosting and platform costs, as well as related services such as CDNs
- Marketing costs: any spend you might have relating to promotions such as Adwords costs, display advertising, email marketing and items such as affiliates etc
- Service costs: if you've outsourced any work or employ staff you need to include these costs in your spend
- Cost of goods sold: if you're selling products or services, it is incredibly important that you include the costs of these in your figures. It is best practice to include the costs at the point of sale rather than when you paid for the inventory.
Note that these are monthly costs based on your historical records. If you have one-off costs, you should highlight these in your listing description so that buyers can adjust for this.