Very expensive. About £20-30 for an hours joint lesson, you may find a riding school that does it cheaper but the standard of horse welfare will be pretty low. You then talking jodpurs £30, jodpur boots any where upwards of £30. Hat £60 must get one with a kite mark safety standard. Body protector £60+ again. Then if she does get into horses and most do, she will beg you for a pony, which you buy that will set you back £1500 at least for a good one, then you talking £400+ a month on keeping it, livery, food, vets bills, insurance, rugs (£70 each properly need at least 3 of different types), riding lessons (please don't stop these if you buy a pony) shoes, hay, tack, (your talking £500 for a saddle) bedding and then all the cute pink things that they "need" too. But the worst is the time spent them, you need to do them twice a day every day, 30-1hr in the mornings, again in the evenings then an hour plus for actual riding.
But it's a great "hobby" lifestyle choice, they do take over your life, and you do get a massive amount of love and fun from them too, but go into it with your eyes wide open.
Oh and don't buy a second hand riding hat or body protector, you don't know if they have had a fall in which case there safety foam will have been burst and therefore useless