As the person leasing in the plane, or leasing it out?
It's fairly easy to work out for the person leasing in. Simply multiply the daily profit by 24 - giving you the monthly profit - and compare it to the leasing fee (which is always monthly). Maintenance is paid by the owner, so you need not worry about that. The only other cost is gate fees - which are fairly minimal, and can be estimated based on which airports you fly to. If you're calculating them, consider that firstly they are monthly, and secondly the fees can be "shared" between aircraft if multiple ones are using a single gate.
In terms of leasing out a plane, you can't really calculate anything because the individual maintenance fees for aircraft aren't given (due to the way maintenance is calculated). You need to consider two things: the duration of the lease and the amount you're charging.
I tend to go for 10% of aircraft value and generally not much more than 5 years (i.e. ensure it is off lease at around 5 years - so if its 3 years old already, only offering it on 2-3 year leases).
You may also wish to try the higher turnover option - leasing aircraft for more like 2-3 years, and selling them second hand as soon as they are returned. You will often be able to get a decent amount of their value back (esp. in larger public worlds). As a rule, I assume that people will keep the lease until it expires - not that they will return it early - and plan accordingly.
Leasing in planes is a fairly risk free venture - as you have minimal investment required (less than buying planes) and you can get rid of the planes whenever you want. By contrast, leasing out planes is fairly risky - with considerable upfront and continuing costs - and no way to stop those costs other than waiting out the duration of the lease. It is quite easy for planes to become a burden (i.e. cost) for you if you don't choose the right conditions early on. That said, it can be a easy way of making money (no continual editing of routes!), if you get things right, and can provide some more interaction with other players in the game.