If you run a business, you probably use Microsoft Office. And if you need to buy a single copy, it ain't cheap. Figure on paying £219 direct from Microsoft.
(Versions designed for home use are much cheaper, but the catch is that the software's licence agreement says it can't be used for business purposes.)
It is possible to save a bit on this by shopping around. At the moment, you can pick up the exact same package - Microsoft Office Home and Business 2013 - for £178.99. That's a saving of £40 on the Microsoft price.
For that you get full copies of Microsoft Word, Excel, PowerPoint, Outlook and OneNote, which are all the major office packages you're likely to need in your day-to-day work.
If you're planning to buy, just be aware that all Amazon will send you is a card containing a licence key for the software. There's no disc included, so you have to download the software online, then activate it with your licence key.
It's probably one of the more expensive pieces of paper you'll ever buy, but if you need Microsoft Office then it's money well spent.