In the horse world and especially with lessons - you get what you pay for. This woman trains with Jan Ebeling (it says so on her website), full training at his place is $2200-2500 US per month (not sure what one lesson is but I would imagine not less than $120/45minutes). Jan is a wonderful coach and you will basically getting his knowledge second hand (along with many other people's knowledge) through this woman for the bargain price of $50! This woman has payed hundreds of thousands of dollars to get where she is and you're thinking $50 is steep?? I think it's very reasonable, personally. You also have to keep in mind that many dressage coaches although they schedule 45 minute lessons will teach over that if they feel they need to.
First thing is first I would go and watch some lessons and see if you like her teaching style and if you learn anything from auditing - you should.