I'm in a similar position to you. Have 2 iPhones an iPad and an Apple TV in the house all of which need to stream media from my PC.
I've literally just built a Gigabyte Z77N-WIFI with an i7 3770K and 16GB of Memory for £412, however that didn't include a PSU, case or operating system as I already had all that.
I considered a Mac Mini but I just couldn't justify the cost considering what I could build myself for a similar price. I also wanted to be able to play games on this rig, so required the ability to drop a GPU in at some point.
If this is just for HTPC use then the Mac Mini is more than sufficient and is an incredibly small and good looking little box, but that is what you are paying for, looks.
How are you going to be using it?
Does it need to contain all your hard disks or do you have another PC with all the storage and media on?