Considering servers have a running cost (electricity, bandwidth, hardware, maintenance etc...) and considering the average live span is around 80 years.
We could do this for 40 * 12 * 80 = 38 400 GBP
If you have any questions, please don't hesitate to ask.
* Of course, this service is non transferable and only usable for the original owner.
I think Eoin@ClanSlots makes a good point here. We really need to know how long you intend to keep the server up as it costs us (as a game server provider) a significant amount of money to keep our servers online, staff online to help you, and the extra services we provide. A 50 slot server isn't a small server by any means as most servers rarely get over the 20 player mark unless they just have a great community base. The resources required for a 50 slot server PLUS the plugins your more than likely to want is going to really drive the price for us up, and this is from a monthly billing period perspective.
So for us to issue a one time payment for a server like this really isn't going to be cheap or if you do find someone willing to do this there is no guarantee that the server your paying for will be of premium quality. That said your likely to pay something starting in the three digit range ($100.00+).