HTPCs don't need to be expensive to deliver snappy performance, 1080p, and hd audio
The expense depends on what you need / want. Do you want it to game? +$(hardware) MadVR? +$(gpu) 3D or full BD menus? +$(commercial player + win license) Do you need it to double as your server?
If you don't already have a server, or intend for the htpc to be a "server/htpc" then you'll need to concern yourself with form factor and hdd cost (and you should consider a quad core if you have a plethora of tablets, samsung tvs, smartphones, rokus, etc that you want your media transcoded to)
If you already have a server or nas, then you can just focus on what you want it to look like vs how cheap you want it to be
I think that a ML03B, picopsu, this motherboard/cpu
, and a stick of ddr3 ram makes a great htpc that you can boot from a usb3 drive. It should be silent as well, for around $200 new
If fanless isn't an interest, an matx+pentium combo would be a good balance of power while keeping cost down. You could definitely transcode at least one full BD rip while using the htpc at the same time. The B85+G3258 was $110 recently, which would only be $40 more than the fanless Q-1900 I linked above. However, when you add transcoding and a beefier processor to the mix, it's easier to just use Windows. Which concludes that you need a license, need more ram, need an SSD, and you get away from "just htpc" which can make great use of OpenELEC and keep costs down.