You can have a look at this demo showing similar calculations client-side. Basically OnBlur of the RadTextBox you can calculate the new value and set it to the corresponding cell of the grid. You can refer to this help article showing how to access cells client-side. There you can also format the cell (including fore color) by a certain criteria. You can also format the appearance of the negative and positive value if you use RadTextBox in a TemplateColumn (using EnabledStyle-ForeColor and NegativeStyle-ForeColor properties), it also allows currency formatting. When the RadTextBox is read only this can also be an alternative to the standard GridBoundCoumn that does not provide such special formatting.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!