I've been building them. Here's the typical mid range.
AMD 3500+
MSI NeoPlat 4
SilverStone LC16M HTPC case
Antec 500w Smartpower PSU
1GIG Value RAM
1- 80GIG Seagate SATA
1- 300GIG Seagate SATA
1- Plextor PX-716AL/SW slotloading multidrive
x-fi xtreme music
ATI X1300
Hauppauge WinTV-PVR-500
MCE-Kit
Windows Media Center 2005
You can go lower on price in every category.
This one is like $1400+