Wel as you have rated my earlier reply I assume you've read it and as you're not saying which I will answer both.
I have had horses for nearly 40 years and my ways are still the old ways I suppose..get the horse used to having its mouth/teeth checked so you dont have problems when someone comes to have a look...as to how much in millimeters - only as much as you can feel is sharp on the back teeth when you put your hand in and run your fingers along the back teeth so that the animal does not get a sore mouth, but in the case of an older animal it may have to be a bit more if they are quidding badly. Teeth should rarely need rasping if the animal is under 6 years old.
How much - financially...up untill about 5 years ago our horses teeth were always rasped by the farrier for no extra charge..years ago farriers could diagnose limb injuries better than vets too and ours was Army trained and knew as much if not more than the local vet. Nowadays our vet looks at them as a matter of course if he has to call for any reason and just charges a minimum over the call out. If you get an equine dentist be prepared for about �80 depending on the area you live in, how far they have to travel and how many horses they look at whilst they are there. It's an imprecise answer but the cost will vary massively from area to area, so its better to phone your chosen professional and ask. Hope this helps- Trudi :o)