My mare's had it done a couple times. As for evaluating, my vet has this little plastic dowel thing she runs along their spine and joints and somehow determines something is out of place. It's really cool to watch. If she doesn't use the dowel, she uses her hands and massages around. It's fun to watch too, as they kind of make the horse crack their back and neck themselves. Particularly the poll. I have no idea what she does. She brings their head down to the ground to stretch or a second, then pulls their nose back and does something so the fling their face forward and head up. It cracks their poll and makes a really loud pop.
I assume the price changes regionally. My vet charges.. $115 for it. Something like that. The other place around here that does chiro is like $220 or more. It was worth it the first time I had it done. Abby had her hip out and was clearly uncomfortable loping. If she tried to out in the pasture it was ridiculously stiff and awkward. If I asked undersaddle, I got bucked. Vet popped it back and now she lopes like there's no tomorrow.