Product option prices and cart display

Hi
Am currently using the default template version 2.1.0.2 and have a query. I added weight as an option in the products and I still keep running into this problem.

Eg if my product is a bag of flour with the weight options as 500gm, 1kg , 2kg and 5kg bag and appears as follows.(I have indicated the actual prices on the side for your view to elaborate further)
Option 1 500gm(-$50) -actual price is $70
Option 2 1kg(-$40) - actual price is 80
Option 3 2kg - actual price is $120
Option 4 5kg (+$300) - actual price is $420
The 2kg bag is my initial price at $120
Ideally if i want to select more than one option lets say the the 500gms and 1kg bag the total order cost should be $150 in my cart. However i can only achieve this if I select the options separately and add them to cart individually. If i select both options simultaneously and add them to cart in one click i get a total order cost of $30 which is totally wrong.

I wish to have this fix
The options show the exact/actual prices of the product i.e
Option 1 500gms ($70)
Option 2 1kg ($80)
Option 3 2kg
Option 4 5kg ($420)
And also be able to select multiple options and add to cart with one click and end up with the correct total order cost.
How do i fix this?

Best solution is to start with your base product price as zero

Then set the option prices as the price for each opton exactly.

okay, but how would i do that and still show the 2kg bag and its price on the product category page before clicking the add to cart button which redirects you to the actual product page where you can view the options?

That's not going to be possible if you are also allowing people to select multiple options and wish to get the correct price total.