Ya get what ya pay for in squirrel calls... I prefer the 50cent squirrel call...

The 50cent squirrel call has a lot more volume due to the size of the coins, and can replicate more sounds than the 2cent call. Instead of using the pillars on the Lincoln Memorial on a penny, you use the reeded edge of the quarter.

In addition to the nut chewing sound, pinching the quarters sandwiched between your thumb and forefinger, then sliding them against eachother and occasionally clicking them like castanets also produces a chirping and barking sound that's surprisingly accurate (with a bit of practice). To be honest, I've found that nickels make the best sound for this.

A "25cent squirrel call" that my uncle used to use was a rubber super ball. Depending on the pressure used, you can make a nut chewing sound up through a barking sound by pressing the rubber ball against the front edge of your teeth as you make a chewing motion. The ball will squeak and chirp against your teeth. I suppose all of those quarter machines outside the bowling alley are more than 25cents these days though.