$74 after shipping? Newegg will sell you a motherboard with a dual-core x86 CPU for $50 ($32 after rebate). With a new AMD AM1 platform, you can get a four-core x86 and a motherboard for $69. Raspberry Pi is a nice thing for $35. For $75, much nicer alternatives exist.
Actually, I hadn't thought about this. My uncle's in need for a media center (he's already got the home theatre and the forty-something-inches LED T.V. and I was thinking of gettin'im a RPi with the rest of the fancy stuff but you totally would get waaaay better performance with a little bit more "expensive" board. The last time that I checked for this kind of solutions I was fancying some Zotac box but that one was…
On x86 front, if size matters, you can take a look at this: http://www.newegg.com/Product/Product.aspx?Item=N82E16813135... ($32 after rebate). It needs a stick of alaptop RAM and a laptop power brick. You can load XBMC from a USB stick.
If size doesn't matter, look for AM1 CPU/motherboard. CPU will run you $31 (http://www.amazon.com/AMD-Sempron-2650-1-45Ghz-AD2650JAHMBOX...) and motherboard, $34 (http://www.newegg.com/Product/Product.aspx?Item=N82E16813157...).