Err, like a spreadsheet or something?
You could write a script.
Or copy your table to a spreadsheet, do the math, and copy it back.
Perhaps you could share some more detail about what you really want to do.
Screenshots always help with confusing questions.
Yeah, no, sorry. I suppose there might be some 3rd party plugins that handle this through spreadsheet/database integration...
And like I said, you could automate it through scripting.
I have never tried scripting in InDesign. I think I will try that - some day
There's a few ways to do it, have a look at this article here, but particularly at the comments, there's a script in there that might do it. Otherwise you can buy a tool that does it here. Word of warning though, it's not as simple as a spreadsheet to use, I downloaded a trial, tried for about 20 minutes and then decided it was going to be much quicker to copy and paste between InDesign and Excel.
One of the features of Smart Styles is to add calculations like Sum to tables.
You can download a fully functional 30-day trial here:
Following a plea for help from me Peter Kahrel has combined and updated a couple of defunct older calculation scripts to create a very flexible and comprehensive Price Adjuster script (works on non-currency figures as well). Multiply, divide, add or subtract on a selection, story or document, with or without currency symbol and thousands separator and with variable decimal places in the output. You can apply a color to changed data to make checking easier (you can delete the color later). Available from http://www.kahrel.plus.com/indesign/price_adjuster.html.