These devices actually fall into 3 categories. The basic process is active noise cancellation, and there are a number of makes (Sony NC5/NC11/NC20), Panasonice, Jensen, Philips etc that give about 10db for frequencies below 1000Hz. These are usually available for under $100, and are without a doubt the best value in the market. in terms of noise cancellation, these are all about the same, and some can be had for a little as about $30. I happen to like the Philips product, the bigger ear cover gives you a little better passive noise reduction.
How well you can cancel the noise depends upon how fast you can process the input, and how accurately you can produce the anti-nose at exactly the right phase. 10db corresonds to a phase error of about 6 degrees. 18db is about 2 degrees.
A great deal of aircraft noise is in fact low frequency rumble, so these actually work very well on jet aircraft Aircraft. Unfortunately 10db is just about worthless on a light piston aircraft.
The next step up use better processors and better alogorithms. The Bose QC series fall into this category, and they are good for about 16-18db at up to 1000hz.
Expect to pay $200-$300.
The last ones get kind of exotic. The mid priced noise cancelling Aircraft headsets like Lightspeed are very similar to the Bose QC technology. The high priced stuff like the Bose X, Telex ANR-1D will run you close to a $1000.
They add a feature called predictive noise cancellation, and while they typically reduce noise actively by about 18db, they can do far better with predictive cancellation.
For example if you are flying an aircraft with a 3 bladed prop turning at 2400rpm, there is a huge noise spike from the prop at 120hz, and unless the engine rpm changes suddenly, it will always be there. Over a period of seconds, these top end units will lock onto the prop noise, and nail it to the point that it actually disappears as far as the wearer is concerned. Going from 102db in the cabin to below audio threshold is something that has to be experienced to be believed, in fact the biggest problem people using these headsets often have is remembering to speak up enough to actually activate the intercom! BEtween active cancellation, and passive isolation, these headset can reduce prop noise by more than 50db!
BTW,earplugs do a very poor job with low frequency. The bind is a lot of percpetion of low frequency is via bone conduction, so an earplug doesn't help much. You really do have to cancel it to make it go away!