it is easy:
-first make your schedule
-assign costs to your activities either by putting in fixed costs or by resources
-go to task usage tab
-at the right section of the screen you will see some rows
-right click there and make sure you add cumulative cost row
-you will see that the cumulative costs distributed from left to right all the way from the start to the end for each activity (with respect to your project calendar span at the top)
-then all you do is to paste the top row which is the calendar to excel and then paste the cumulative row to the excel exactly under that first row so you have two rows under each other in excel (make sure dates and costs correspond as in ms project)
-then you can make a graph in excel showing an s curve with one axis being the time and the other cost
hope this helps