I don't think I know anybody who can use the numpad on their laptop any more effectively than they can the top key row. If you have to do number crunching it is useful to be able to do all you input with one hand but you quickly find it is either not very fast, or if you do it fast it becomes quite painful.
Enter the USB calculator/numpad. This (Canon DK-100i CP Mini Handheld Laptop Calculator) is the one I got and it has paid for itself a few times over. I would recommend this model only for casual use. It is lacking a few things that would make it more useful. Namely a tab key. Also the keys are a little too soft to press and the reference nub on the '5' is a little elusive.
Anything more intense than casual use and you should look to something a little more expensive.