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.
