A difficult conditonal calculation

Hi.
It's the look of my Excel file:

Item 1----Amount
Category---Details: ...
Item 2----Amount
Category---Details: ...
Item 3----Amount
Category---Details: ...
Item 4----Amount
Category---Details: ...

As you see:
- A1 is the item's name; A2 is the price/amount/cost/value of the item
- B1 is the category the item belongs to; B2 is the description of the item.

Now I would like to do some math which are the following:
1) If the category type is income, add all of them
2) If the category type is one-off expense, add all of them & put a minus sign
in front of the value
3) If the category type is daily expense, add all of them & times 30, & put a
minus sign in front of the value
4) If the category type is Asset A, put each amount of the item in this
calculation: (Amount-1000)*3/5
and so on

How can I achieve this?
Thanks a lot.