They do make sound baffles for ducts very similar to a muffler for a car where the air is redirected through several chambers while still allowing adequate airflow but you have to replace a section of the duct.

http://www.acousticalsurfaces.com/hvac/duct_silen.htm

So if you have easy access to the duct, yeah, but cheap? I doubt this is cheap. If there was a cheap way to reduce noise via duct work everyone would have it already. Putting a piece of insulation in your duct, as someone else suggested, is a pretty horrible idea. You'll be reducing sound sure, but also airflow but more than that you'll be blowing fiberglass into your duct system and breathing that stuff in.

If I were going to try to do this on the cheap I'd get a block of styrofoam the size of the duct and 4"-6" deep, cut some holes/channels through it, spray it black so it won't be obvious from the vent, and stick it in the duct. You're going to be reducing the air exchange in the room for sure but it's better than fiberglass blowing around your house.

You're creating a hazard for dust build up (allergies? Fire?) and God knows what other types of hazards by intentionally restricting the air flow through the duct but. The material can mitigate a lot of that risk.

Oh, fucking pool noodles. Even better, they come pre-holed. Grab a pool noodle or two (or three? Four?) And cut them into.... well whatever size you want say a foot, assemble the size of the duct, tape them up. Bam. Longer lengths, less air flow, less noise, shorter lengths, more air flow, more noise. Pool noodles are cheap. Fuck it. Just fill the whole damn duct up with pool noodles first and see what it sounds /feels like and start cutting them down from there if you have to.