Calculating Cost Based On Several Factors

i. I currently have a spreadsheet which is used to forecast resource cost for a project. The forecasted cost is calculated on a few factors - rate, allocation, contract start and end date, and expected days worked per month. One of the mods actually helped me out with this a few weeks ago.

I now have been told that there is a possibility that certain resource costs may change in the new year and that will need to be reflected in the sheet whilst keeping the historic information.

For example - XXX has a rate of £200 p/d, allocation is 1, working 18.83 days p/m and is working from 01/01/09 to 01/06/09. The current formula will work out his cost per month until contract end. Now say his rate will be changed to £150 p/d from the 01/03 and all other info remains the same, I need the sheet to calculate his revised cost from 01/03 onwards and not change the calculation previous to that month.

Now Ive actually managed to figure that part out myself by adding in two columns (over-ride rate and over-ride date) using a nested IF statement. The only problem is that if the new rate starts mid month then it will still calcuate the original amount for the full month and the revised amount from the next month.

Edit - Also, could someone advise as to how do I remove my old attachments as I have almost used up my allocation.