How much a silent PC costs depends on your requirements. The Raspberry Pi range offer lots of possibilities, and are a fantastic way of learning about computers and Linux. There's an amazing number of projects you can build too.
For tasks which require more power, there are many low TDP processors which can form a silent PC with plenty of horsepower. Don't forget, if you want true silence, you'll have to dispense with your keyboard and mouse; a touch screen silent PC might just be the ticket.
<A HREF="http://www.linuxlinks.com/article/201512060358549/SilentPCs.html">Should I buy a silent PC?</A>