How airlines are calculating crew member's payroll?
As mentioned in the comments, this is different per airline. I'll list some of the options used, though there may be airlines out there with their own unique system not included in this list. Note that I use Flight Attendants as examples, as their payroll information and training requirements is a bit more "open source", but these pay methods carry over to anyone working in the aircraft.

Hourly:

This is by far the system of choice for the larger airlines, as they are more easily able to track performance and efficiency statistics (hours worked v hours needed v pay per hour), as well as the fact that it often ends up being cheaper. Flight Attendants have seen remarkable raises in recent years for most companies, and can make anywhere from $25-60/hr, depending on how long they have been working with that particular airline, or in that particular occupation. This provides an incentive for sticking with your current employer.

Static Salary:

A static salary is similar to what you would find in a regular job. You have a minimum number of hours expected of you, and you get paid a flat rate. In the interest of keeping your job, you would be expected to work many more hours than the minimum, as cabin crew understandably work different amounts each month.

Dynamic Salary:

A dynamic salary is a mix of the above two, and is more of a European method. This type of pay expands or shrinks depending on your hours. There is a maximum number of hours you can work, but a relatively low minimum. If you hit the maximum number of hours, or more, you will receive 100% of your salary. If you work half the number of hours, you receive 50% of your salary, and so on. These salaries are typically much higher than a static salary, but it averages out to about the same as a static salary. 

Trips for Pay:

TFP is not the easiest way to do it, but gives the airlines a way to pay the lowest wages! You get paid "per trip", as long as the trip hits the minimum distance to count as a trip. If not, you earn a fraction of "per trip" pay. Depending on the airline, a standard trip is typically between 200-300 miles. If the trip is longer, you get paid an additional rate per mile. This system also tracks time that is blocked off for your duties, so you will still get paid if you are waiting for your next aircraft to arrive, but it is a much lower rate.

Extra Pay:
On top of all those methods of pay, nearly all airlines provide some form of additional pay that is added either per trip, or per month. This can be for special skills or training that a flight attendant has received, or for working through holidays or red-eye flights.

That said, I spoke with an old 747 pilot in my family, and he mentioned that holiday pay is not all it's cracked up to be. He was used to them calling in  pilots to work every Christmas Eve, so he ensured when he got out of work on the 23rd, he would take a swig of brandy as soon as possible so that they couldn't legally force him to work extra hours!