I'm trying to add some additional data to dropdown options so I can use them in JavaScript. However, I can't seem to be able to use the $BasePrice or $Blah variables in the template. Anyone any tips?

The code I'm using is:

DropdownExtraDataField.php

<?php

class DropdownExtraDataField extends DropdownField {
	/**
	 * @var boolean $extraData Associative or numeric array of extra data for all dropdown items,
	 * with array key as the submitted field value, and the array value as a
	 * extra data included in the interface element.
	 */
	protected $extraData;

	/**
	* @param string $title
	* @param array $extraData
	*/
	public function setExtraData($name, $extraData) {
		$this->extraData[$name] = $extraData;
		return $this;
	}

	public function Field($properties = array()) {
		$source = $this->getSource();
		$options = array();
		if($source) {
			// SQLMap needs this to add an empty value to the options
			if(is_object($source) && $this->emptyString) {
				$options[] = new ArrayData(array(
					'Value' => '',
					'Title' => $this->emptyString,
				));
			}

			foreach($source as $value => $title) {
				$selected = false;
				if($value === '' && ($this->value === '' || $this->value === null)) {
					$selected = true;
				} else {
					// check against value, fallback to a type check comparison when !value
					if($value) {
						$selected = ($value == $this->value);
					} else {
						$selected = ($value === $this->value) || (((string) $value) === ((string) $this->value));
					}

					$this->isSelected = $selected;
				}

				$disabled = false;
				if(in_array($value, $this->disabledItems) && $title != $this->emptyString ){
					$disabled = 'disabled';
				}

				$extraDataValues = array();
				foreach($this->extraData as $key => $v) {
					$extraDataValues[$key] = $v[$value];
				}

				Debug::Show($extraDataValues);

				$temp = 					array(
						'Title' => $title,
						'Value' => $value,
						'Selected' => $selected,
						'Disabled' => $disabled,
						'Blah' => 'Test'
					);
				Debug::Show($temp);

				$options[] = new ArrayData(array_merge(
					$temp, $extraDataValues)
				);

			}
		}

Debug::Show(print_r($options, true));
		$properties = array_merge($properties, array('Options' => new ArrayList($options)));

		return parent::Field($properties);
	}
}

<select $AttributesHTML>
<% loop $Options %>
	<option value="$Value.XML"<% if $Selected %> selected="selected"<% end_if %><% if $Disabled %> disabled="disabled"<% end_if %>>$Title.XML ! $BasePrice $Blah </option>
<% end_loop %>
</select>

Form:

	public function Form() {
		$fields = new FieldList(
			DropdownField::create('ServiceType', 'Service Type'),
			DropdownExtraDataField::create('PaperTypeID', 'Paper Type', PaperType::get()->map('ID', 'Name'))->setExtraData('BasePrice', PaperType::get()->map('ID', 'BasePrice'))
		);

		$actions = new FieldList(
			FormAction::create("OrderForm")->setTitle("Submit")
		);

		$required = new RequiredFields('Name');

		$form = new Form($this, 'OrderForm', $fields, $actions, $required);

		return $form;
	}

Have you tried simply

$field->setAttribute("data-$key", $value);

Thanks Pyromanik, that only adds it to the select though.

I've found what I did wrong and created a module to add data attributes to options of the select. For those who are looking for the functionality see: https://github.com/marijnkampf/silverstripe-dropdown-attributes-field

Oh I see, sorry I misunderstood the question.
Good to hear you got it working, and thanks for updating the thread with the solution :)