I'd start by creating resources on the resource sheet. You can create as many resources as you need and enter a cost for each resource. You can also have resources with maximum units over 100% if you are working with groups of resources.

Then assign resources. Project will calculate the work and the cost based upon the number of resources assigned and the cost for each resource. The work will automatically roll up to the summary task level as will the cost.

If you save a baseline you will have the original cost preserved and then you can start tracking and project will calculate actual costs.

From your posts, it sounds like you are attempting to create custom fields to perform the calculations that Project will do for you - if you add the information it needs to calculate.