I was raised wearing dresses/skirts. We did everything that you can do on a cattle farm with ankle length dresses. You know you have the knack down when you can climb through a three strand barbed wire fence and not get you skirt or hair caught once!
I rode horses and broke colts while wearing dresses/skirts. Eventually my Dad started to let us girls wear culottes (split riding dress/skirt). We always made our own dresses, so as far as buying and costs go I can't help you. But I will say, it is possible to ride in a dress. I find it a lot easier to ride in pants, so since I moved out from home, that is what I do.
Here are some pictures of my and my sisters...
The style of dress we liked best
Riding bareback in the same dress
I'm in front riding in a dress with a big and billowy skirt, my sister in the rear with a narrower dress
Riding in an Australian saddle
Now here a couple of the culottes we used
Same pattern, my sister
A pair of culottes that I sewed
Same pattern different fabric
We rode western mainly, some bareback and some aussie. Your worst problem is going to be the dress getting caught on the cantle (sp?) while swinging on and off. Oh, and when you get up into a canter or gallop you have to make sure that you keep tucking the front of the dress in and around you legs.
We would wear leggings or longjohns under our dresses for comfort, modesty, and coolness.