Advice on the Best way to approach multiple lists that need to reference and SUM.

I have 3 different SP Lists, that I would like to reference each other and also sum various columns.

• Charge out list - The Hourly cost per employee.
• Time Sheet List - Where the employee logs hours agaisnt a reference, that is linked to either an internal or external rate in the charge out list.
• The Reference List, the work to be carried out that Time sheets are logged against that will hold the total sum of hours times the Hourly cost.

I would like to have users enter time sheets, the rate is then caluclated and the hours are then multiplied, then using the reference fromt he time sheet add those figures as a total to the Totals list.

Any thoughts on the best approach to completing this? I am open to changing the whole system if needed.