I wrote a previous version of this extension for extjs 3, basically is the same code, just adapted to work with current version 4.
The extension extends from Ext.form.field.Number and add these properties:
fieldStyle: 'text-align: right;',
1. Display a given currency symbol, default value is null.
2. Display a thousand separator, this can be any character including blank space (' ') default value is ",".
3. Force decimal precision to display all decimals from "decimalPrecision" property.
If you set thousandSeparator config option to "." automatically decimalSeparator will be set to "," unless you also set decimalSeparator explicitly.
I didn't remove the trigger so you can still click up and down to increase or decrease the value. It will be nice to know the opinions of this one.
I tried to use Ext.util.Format but it doesn't allow to apply all the options the extension provide, so i still use my own function to format the value. One property that is missing from the extension is "currencyAtEnd" requested before, i couldn't add it today.
Full size image of combinations can be found inside the zip file.
For those just use USD currency formatting can take a look to this extension.
Previous version for extjs3 could be found here.