Difference between two dates and taken calculated results with other cells in error.

Hello to all.
I have a task that I'm almost finished, but there is a point of it, that does not crack, but only text and I do not mind coming how to finish it. Here's part of it - what you do not know how to finish.
The date is taken to the function of the "Date" (blue).
Number of nights is the difference between the final and the date. If no end date to display zero (0) (Purple). - >> So far I think I did.
The big problem in plain text - the amount of room (price multiplied by number of nights) is determined by two variables: the base price (A12) and the coefficient of categories (A13), which are provided in the individual cells under reference. Room rate is defined as the daily price (A12) added coefficients CATEGORY (A13) multiplied by different categories (A4: A8) (napimer for the third room category will receive price 30 +3 * 5 = 45 lev For .. 1 and 2 have not changed the price in the third person price increased by 50%.
If done correctly in cell L13-must be obtained result 3495.00 if another means something is wrong.
Very important poverty should not be changed table - numbers and dates in cells otherwise you will never get the result. Thanks to everyone responded to help.