So if I were to suddenly need to add 45 people to the office. This would be cheaper than buying a whole lot of licenses? If so I'll keep that in mind if my company gets awarded a big tender in 2 months.
No. Microsoft Volume Licensing (MVL), of which there are many types, is not the cheapest way to buy licenses (depending on the total number of users); OEM/DSP is. Retail is the most expensive, and MVL sits in-between the two. One of the biggest advantages of MVL is that you can install the license on many machines (although only one at a time obviously), unlike OEM/DSP which lives and dies on the machine (Full Retail packages are also not limited to a single machine). The one disadvantage of MVL is that you need to buy 5 licenses to enter the program....
MVL also allows you to buy Subscription Advantage, pay off all licenses over a period of two or three years, and it also offers packages of licenses in a single product, i.e. CoreCal which has a Sharepoint, Exchange, Server, Forefront, System Centre and OCS license included. You also get better pricing models as your number of users increases >250 and you can enrol in the Enterprise Agreement. Lastly, you also have the True-Up option, so you could potentially grow, and then shrink before the True-Up period commences
Short story, MS licensing is the MOST complicated form of licensing known to mankind....