minimal. probably $150-200 for now.
i checked craigslist for some amd a6'es and a8's... no dice. i couldn't even find any i3's. same goes for atfs/t.
what os/front end do you recommend?
OK, that probably settles that... and you don't need a lot of power (like an i3.) My HTPC is powered (overpowered! ) with the Pentium G620... it doesn't break a sweat (note that I do not encode on it, I use my i5 PC for that.) You might be able to find a SB or IB Pentium or Celeron/mobo combo in FS/FT if you hold out, but the C2D will work if you don't mind spending money on an obsolete board.
If you are a Windows guy... I like W7 because it comes with Windows Media Center (WMC,) a very capable front end with ease of use. W8~ does not come with WMC.
If you want to try something else, everyone raves about XBMC, and there is a version of Ubuntu that has XBMC built into it (as I understand it.)
What kind of case are you looking for? A traditional HTPC case can get expensive... I mentioned the HDDs... some smaller (less expensive) cases only hold 2 HDDs, so make sure you look at what you need. Although my HTPC's PC case wasn't cheap (it fulfilled a specific size requirement and had sound damping) if it's all the same you can get a very nice PC case typically for less than a traditional HTPC case. Just depends on what you want.