Not as much as you might expect and actually less than X10 costs for the dimmers.

Enttec USB to DMX controller: ~£100, but you can get controllers for much less
Chinese 4 channel DMX dimmer packs: ~£56 from CPC (http://cpc.farnell.com/jsp/Audio,+Vi...sp?sku=DP27530)
Sundry items such as cable, plugs, XLR connectors etc.: ~£50 to £100
X10 bits still in use: ~£150

I use about 6 dimmer packs at the moment, so ignoring computer costs, core components are maybe £500 to £600 to control most rooms, hallway and porch. I need to get a few more packs fitted to complete the job and get the last of the X10 dimmers out.

There was also the cost of rewiring the building plus new lights everywhere, which made it more easily achievable and multiplies the cost a few times, but this had to be done anyway as the whole place was being renovated.

Ignoring X10, there are other ways to retrofit remote control lights such as Rako controls, and although not cheap, you can do it one room at a time without needing a computer or rewiring the whole house, and get a reliable and professional solution. There are many players in the home automation market, but it is fun to do a DIY solution and ultimately more flexible.

Automated lights in homes are nowhere near mainstream yet, and I think this is a shame. But over time it's going to become more common. The advantages include the convenience of automatic scene setting, smarter lighting control to turn off lights in unoccupied rooms and on when you enter, operation when away from the home to simulate occupancy, easier building wiring as you don't have to run mains to wall switches, and over time there may be reduced running costs too. We will also see more use of LED lighting for much lower power usage and increased safety over the popular choice of halogen lights (one cause of house fires), and 100,000 hour lifetimes instead of 1/10th of that or less with halogen and other choices.