Entracte Audio Powerline Filters

Capacitance/inductance. Those are the most common ways to filter power. Don't worry, many other types have been made, but if you want a quick, inexpensive circuit that's it. You can a few film capacitors of different values to cover a wider frequency range to filter. A good metal housing is probably safest way to go, and it will provide shielding, at least for your capacitor circuit. 

 I don't know what the final cost of manufacturing the least expensive model will be. I would bet around $200-250, depending on AC outlets used, and few other factors. The capacitors will not run a lot of money.

  They will likely not hurt the sound and will reduce noise, but at the price there are better options.

emil,

Thanks for reading my post. What I should have said is that this product could be built by most in the manufacturing business for a lot less. Sorry that I cannot give a specific example here. 

 I am the DIY type, so sometimes I will explore a product that I need (such as a speaker, preamp, amp, etc.) and then proceed to make one to my liking.