config.yml

Injector:
  CurrencyField:
    class: FixedCurrencyField

FixedCurrencyField.php

<?php

class FixedCurrencyField extends CurrencyField {

	/**
	 * @param mixed $val
	 * @return $this
	 */
	public function setValue($val) {
		if( ! $val) $val = 0.00;

		$symbol = Config::inst()->get('Currency', 'currency_symbol');
		$this->value = $symbol . number_format((double)preg_replace('/[^0-9.\-]/', '', $val), 2);

		return $this;
	}
	
	/**
	 * @param Vaildator $validator
	 * @return boolean
	 */
	public function validate($validator) {
		$symbol = Config::inst()->get('Currency', 'currency_symbol');

		if(!empty($this->value)) {
			if(!preg_match(
				'/^\s*(\-?\\' . $symbol . '?|\\' . $symbol . '\-?)?(\d{1,3}(\,\d{3})*|(\d+))(\.\d{2})?\s*$/',
				$this->value
			)) {
				$validator->validationError(
					$this->name,
					_t('Form.VALIDCURRENCY', "Please enter a valid currency"),
					"validation",
					false
				);

				return false;
			}
		}
		
		return true;
	}

}