Costing database question

I am trying to create a costing quote database. The problem I am running into is the follow:
I have a popup calendar that allows the salesman to enter a work start date and a work ending date, along with a work start time and ending time (i.e - to figure out if they are working eight, ten, or twelve hour shifts) and a box to denote one shift or two shifts per day. Finally there is a box with a drop down menu to choose whether or not the weekends are going to be worked on. Once all the data is entered I need a formula to calculate the following:
- Multiply total hours by an assigned cost to get a total cost. However, it needs to figure out that if there is no weekend worked ( "NO" on Weekend Worked Drop down box) it must skip those days and if the weekend was worked ( "YES" on Drop Down box) then it must multiply the cost by 1.5 for those two days to get the OT cost.
I realize this is a fairly advanced request and unfortunately a bit over my level of experience so any help would be greatly appreciated!
Thanks.