How to make onfocus in custom metabox

I have created metabox with array like this,

array(
'label'=>
'Price Product',
'id'=>
'price',
'std' =>
'default value',
'type'=>
'text'
),

and for case i have created,

case 'text':
echo '
<input type="text" name="'.$field['id'].'" id="'.$field['id'].'" value="', $meta ? $meta : $field['std'],'" size="30" />
';
break;

my question, how to make default value disappears when people click on the box.

Thank you