Nothing here you won't find the answer to in the great book written by Nick :
http://www.wangafu.net/~nickm/libevent-book/Ref4_event.html
1/ You set the range by calling event_base_priority_init().
2/ If you don't set the priority, " the default is the number of queues in the event base, divided by 2."
3/ The lower number is the higher priority.
4/ You should use event_base_priority_init() instead of event_priority_init() which is deprecated. event_base_priority_init() set the number of priority for the base, while event_priority_set() set the priority for an event.
5/ You can't use event_priority_set() on an already added to the event loop (event_add()).
6/ You need to remove it from the event loop with event_del() first.