Steve,

The cheapest entry level system that I could recommend is the 'system 4' from DIYCNC. For the motherboard and 3 drivers this comes to about £113 plus delivery.
http://www.diycnc.co.uk/html/system4.html

I have the previous system 3 and it does everything I want at a good price. I have also had excellent customer service from Roy.

Some would consider the 30v input on the low side, but I find it fast enough for my needs. If you want to move up in speed alot of people run at around 50v, and some at 70v. You will of course need different controller drivers for this which are more expensive.