I would say it's probably easier to do it yourself - at least that way you'll be getting to grips with the tool and there are sufficient mathematical formulae available to do what you want. I assume the majority of it will be multiplication with a repeating table so that people can add further lines.
I process all of my forms via DBXL and into a form library within SharePoint for medical research which includes multiplie lined tables for comorbidity information, etc and it all works really well. Just work out in advance what information you want to display within the library to make sure that you promote this property when publishing the forms to allow people to browse high level information without needing to open the forms, but beware of trying to display repeating items within the list.
Maybe limit it to just total order total, etc - guess it all depends what business process you're modelling and who needs to see what and when!!!!
Ged