Filter custom post-type on custom field value

Hi there,

I've got a little bit of a problem.
I would like to make a front-end filter for one of my custom posttypes created with costompress. The post type has a few custom fields for which i would like to make the filter. For this i'm using:
$args['meta_key'] = $key;
$args['meta_value'] = $value;
This works fine for any sting value in the custom field.

The problem however starts when I'm trying to use $args['meta_compare'] = 'BETWEEN';
I would like to get only the results between two nummeric values. This doen't seem to be possible because it seem i can't set any nummeric (int) values as customfield value.

Anyone a thought on how to tackle this problem?
Thanks!

Kind regards,
Bart Jan Hoekman