1. Upload through Sobi2 Plugin Manager (not Joomla Installation manager)

2. Some Sobi2 Core modification is required (make sure that you remember to redo this for every Sobi2 Update)

/*
* You need to modify a file to make the plugin enable to collect entry adding price for a category. Please Open the file *

* components/com_sobi2/sobi2.class.php. First find the following code snippet around line 678.
*/

/*
if(count($config->S2_plugins)) {
foreach($config->S2_plugins as $plugin) {
if(method_exists($plugin, 'save')) {
$plugin->save( $plugins ,$this->id);
}
}
}
*/

/* Then, replace the above code snippet with the following one. */

if(count($config->S2_plugins)) {
foreach($config->S2_plugins as $plugin) {
if(method_exists($plugin, 'save')) {
if($plugin->name_id == 'Category Pricing'){
$plugin->save( $plugins, $this);
}else{
$plugin->save( $plugins ,$this->id);
}
}
}
}

3. Go to Components >> Sobi2 >> Category Pricing

You will see all your categories there and you will be able to set up price (fee) for each one of them. Use 0 to leave a Category for free.