If you have a table that combines all transactions (accounts payable and accounts receivable), you can view the total in summary function for the field that contains the dollar amount. On a computer the summary function appears at the bottom of the screen. On a mobile device, you have to pull down to see the summary function.
You can then group transactions by month (on the computer) and see the result of the summary function for each group. You will need a formula field to extract just the month and year so that you can group by month.
On the other hand, this only works with historic data and probably duplicates information that you should be able to get from your accounting software.
If you want projected profitability, things get more complicated.
Although this situation seems simple at first, it could easily get quite complicated, depending on several factors.
- Do you charge per day or per month? (e.g. does February cost the same as January)
- If you charge per month, do you pro-rate for partial months for the first and last months?
- Do you know the departure date in advance?
- Does your accounting use an accrual basis where you defer the income from the setup charge over the course of several months to smooth out spikes in income when new customers start?
Depending on the answers to the above questions, your situation might be too complex to answer in the context of these forums.