Hey Dean
This really depends on what you are trying to achieve. If you are looking to
replace the Excel spreadsheets all together then I would say move your cost
estimation features to InfoPath. Unless there is a main reason why you need
Excel to do this cost estimation. Adding the spreadsheet control to the view
is something I haven't tried myself  but could be a good middle ground to get
the best of both worlds, if a little problematic.
Not knowing the inner workings of the feature you are looking to implement
makes this a tricky question.