The WordPress requires the BETWEEN values to be sent as an array.
I therefore modified the call to:
&csrp_meta_key=price&csrp_meta_compare=BETWEEN&csrp_meta_value=100,500&csrp_meta_type=ARRAY
and adjusted the custom-simple-rss.php code:
if( isset($_GET["csrp_meta_type"]) ){
$csrp_meta_type = sanitize_text_field($_GET["csrp_meta_type"]);
}else{
$csrp_meta_type = 'CHAR';
}
TO:
if( isset($_GET["csrp_meta_type"]) ){
$csrp_meta_type = sanitize_text_field($_GET["csrp_meta_type"]);
if($csrp_meta_type == "ARRAY") {
$csrp_meta_type = 'numeric';
$csrp_meta_value = explode(',', $csrp_meta_value);
}
}else{
$csrp_meta_type = 'CHAR';
}