The format method automatically does the commas and periods or visa versa (depending on whether your profile is in the US or elsewhere)

Assuming the $ sign applies to all cases in your org you can simply use

Decimal input = 2000;
String output = '$' + String.valueOf(input.format());

Note the String.valueOf(). This is required because the input variable is defined as a Decimal.

Edit: I noticed a bug with the Decimal.format() method where it will not show any numbers past the decimal point if there are only zeros there, ex. $100.00. To solve this I came up with this method.

private String getCents(Decimal x){
    String y = String.valueOf(x);
    String z = '.';
    if(y.contains(',')) z = ',';
    y = y.substring(0, y.indexOf(z));
    if(x - Decimal.valueOf(y) == 0)
        return String.valueOf(x.format()) + z + '00';
    else return String.valueOf(x.format());
}

Then to update the example above, it would be:

Decimal input = 2000;
String output = '$' + getCents(input);

IsNumeric() method works for only Integer value not for decimal value

like this

String str = '10';
system.debug('===strToDec=='+str.isNumeric());

This will return true.

String str = '10.25';

Decimal strToDec = decimal.valueOf(str);

system.debug('===strToDec=='+strToDec);

String decToStr = String.valueOf(strToDec);

system.debug('===decToStr=='+decToStr);

Run this code in developer console ad check output.

In you case do something like. First convert string to numeric then check and again convert string into decimal for future use

String  str = '10.25';
Integer intCheck = Integer.ValueOf(str);
Decimal decVal = Decimal.ValueOf(str);
if(String.ValueOf(intCheck ).isNumeric())
//use decVal here

Don't understand why you need this.