It depends on how many buttons you want on the switch plate, but it costs around $350 for a '1 channel' switch with two buttons (So it can control one local circuit and another that is somewhere else in your house) and up to $450 for a '2 channel' switch with 6 buttons (It can control two local circuits, with 4 buttons for other purposes). You can get the switches in all combinations - 1 or 2 channel, 2,4 or 6 buttons. The channels can either be dimmer channels or relay (on/off) channels.
You can also get plug in modules for table lamps etc - again as dimmers or relays.
If you want to connect the system to your PC then you also need a wired to wireless gateway for around $230 and a PC interface for $450 (or $650 for Ethernet).
Compared to a $20 light switch, it is definitely expensive.
C-Bus stuff is often listed on eBay and sometimes it is cheap if it is second hand, but generally it goes for close to full price.
Paul