Im not sure on cost as my vet bill hasnt come through yet! My dentist charges around £25-30 per horse, less if theres a few to share his visit. Mine are mainly done by vet so they can be sedated and then the electric float can be used, much better result IMO.
Got one horse who could probably have yearly or less visits, and the other is a 4- 6 monther due to his mouth conformation.
normally is 6 months to a year, dependant on how you think they are doing.
it depends on how their teeth are really. every 6 months , or yearly. my 15year old mare is done yearly with her innoculations.( which saves double call out) vet will reccommend how frequently it needs to be done.
costs about £25, plus call out.