@Stephanie
In terms of costs & expenses related to hosting, I do not consider my mortgage payments as a hosting cost because it would be the same regardless of whether I host or not. I understand this would be different if a host bought a home with the intention to use hosting income to pay the mortgage.
Also since I am hosting out of my home, I would have bills and costs associated to the cost of living that I'd have to pay for anyway such as for cleaning, utilities, garbage disposal and taxes so I tend to look at "increased costs/expenses" due to having 1 more person (= the guest) living in my home, and then the cost/expenses of additional amenities I need to buy and supply and kitchen basics which are shared, and cost of labor that is needed (cleaning, laundry, communication with guests, managing the listing etc.) for hosting responsibilities as what my "hosting cost" would be.
Then in my case, we host longer staying guests (usually staying for at least 1 month) so we do not have frequent turnovers.
Overall in terms of utility bills, comparing months with a long-term guest (3 people) vs. without a guest (2 people) there is a difference of about 10~20% (which is anywhere from about $20~50/month) depending on season. For our first 2 guests, the increase was barely noticeable but since then, we've had guests who stayed home a lot or were excessive users 🙂
Amenities and kitchen basics like soap, TP, coffee, cooking oil etc. I'd consider at most $30/month but very likely less. The most difficult to calculate is the cost of labor (for Henry and me) since we are on-site hosts but generously factoring in a total of 2~3hrs per week between the 2 of us, at minimum wage this would add up to about $100/month for the EXTRA cleaning and laundry we are required to do because we host.
For the additional furniture (bed, desk& chair), sheets & bedding, and guest towels, we saw this more as an investment to start hosting so we consider these items "assets" which will depreciate over time (expected life 3~5yrs) rather than a cost. Fyi, we made an initial investment of about us$700 when we first decided to list our guest bedroom on Airbnb, and I think we've spent an additional $600 spread over the past 2yrs for replacements or additional items for guest use.
So on average the cost/expense of hosting a 1 month stay is about $150~180/month, even if we factored in depreciation we'd still be under $200/month (out of which, we are paying ourselves $100/month), and our monthly payout (excluding fees) for long-term stays have been as low as $610 (when we were first starting out and had no reviews) up to $810 during the summer peak season when the AC is on 24/7. Since we have long-term guests for 9 months of a year, hosting has ended up being a great supplemental income for us.
Of course, Henry (and I) did this math before we made the decision to host, and we looked at prices of similar listings and competitive alternate accomodations (such as guest houses, homestays, budget motels) to see how much we CAN/SHOULD charge and factoring in our estimated costs/expenses we were able to determine in advance if hosting would be "worth it". Since we did not consider long-term hosting at first our estimated profits and occupancy rates were way lower; anyhoo~ long-term hosting has allowed us to reduce costs/expenses much more than we initially planned so it worked out very well for us 🙂
Henry and I didn't use any spreadsheets like @Robin4 but we did sit down and list everything on paper when we had these discussions.