Determine Cost Based On An Item Checked

So I am having problems coming up with a script to accomplish the following:

• I have 6 line items with a check box for each item
• I have one quantity number field
• I have total cost fields

In order to determine the calculation of what goes into the I need to be able to check the box which will take the qty*the amount for the item (in the code).

For instance,if I have a qty of 10, and if I check item a, which is $55.00/month, I want the monthly total to equal the $55.00x10.

I hope that makes sense.