Hey  DDR:  This such a timely post (from my pov). 
This Monday I'm joining a group of 10-12 other women (of a certain age) in a drill team  learning situation.  I'm looking forward to it in order to give me somewhat of a goal in my riding. 
It'll take place every second Monday.  Cost is $30.00.  It's just around the corner from me - so, think a mile or so, and I can use one of the school horses to start with in order to gauge whether or not my horse could deal.  The thirty bucks cover one and a half hours of exercises, my horse, or hers, and hoping for an enjoyable experience.
Generally riding lessons - from reputable barns around here- run around $30-45.00.  East Central Ontario, Canada.