Assuming that PC is powerful enough to act as a DVR (you'd have to run the digital cable adviser program inside WMC first to be sure) your main savings will be in the form of DVR/cable box rental fees.
The city of Pittsburgh makes Comcast publish prices for customers. Those are the prices valid to me. So personally, I save:
Cablecard Credit: $2.95
HD Technology fee: $7.00 (this is if you have their HD equipment in your home)
Anyroom DVR Service: $19.95
two extra Anyroom boxes: $9.25 x 2 = $18.50
So I save about $50 per month not renting their equipment
You will have the PC as your main DVR, but what about other tvs? You have a few options
1) buy more computers for the other tvs
2) buy media center extenders (which the Xbox 360 is and PS3 is NOT)
3) rent additional boxes from Comcast
The 360 has DVR playback capabilities built into by Microsoft to work with Windows Media Center. A PS3 or Wii is irrelevant.
It may or may not be a good solution for you. I guarantee you that if you're at all like me it will become a money sucking hobby, not a way to save money. You will want a solid state drive so the machine loads artwork faster. You will want to backup your movie collection, so a 2 terabyte external hard drive becomes a 12 terabyte server. Eventually perhaps the ability to play 3D movies or launch console emulators from the media center software. Now I can control my lighting and security camera with the software as well. If I had a newer thermostat, sump pump, or alarm system I could control those things too
I suggested it to the OP because it can be an extremely cost effective way to create an OTA DVR system, especially since he said he already had a 360. Somebody with powerful enough desktop pc and a 360 already in their living room only needs a $50-$100 tv tuner to record all the major broadcast networks.