Example form is linked to a database table, but won't be saved in this example.
When you submit the form, the value is presented with debug_dump(), so strings are quoted and array values listed.
<?php
$field = new form_field_currency($form, 'Amount');
$field->db_field_set('amount');
$field->currency_char_set('£');
$field->zero_to_blank_set(true);
$field->format_error_set('Your amount does not appear to be a number.');
$field->min_value_set('Your amount must be more than or equal to XXX.', 10);
$field->max_value_set('Your amount must be less than or equal to XXX.', 9999);
$field->required_error_set('Your amount is required.');
?>