Horses are ingenious animals: it's been my experience that if they can possibly find a way to injure themselves, they will. Usually about the time that you're congratulating yourself on how problem-free your horses have been!!
Vet fees vary tremendously by area. Your best bet for an accurate estimate would be to call a local vet and get a fee schedule. We avoid farm calls whenever possible by hauling our horses to the vet if possible.
Some vet bills can be avoided by proper care, feeding, and vaccinations.
Other times, it doesn't matter what you do. Be SURE and find a good vet and treat him/her well. Same goes for your farrier/trimmer.
As others have said, establish a fund for vet bills. When you do this, remember to include the cost of possible euthanization and whatever it will cost you to dispose of the remains. A lot of people are shocked by these expenses, but it's not like burying your pet canary in the back yard. Hopefully, it won't be necessary, but if it does become necessary, you'll be glad the money is there.
To some extent, vet fees also depend upon how you view your horse. To many people they are pets and adored family members. These people typically have a few horses and use them for recreation. Others view them as livestock, valuable as long as they are useful for performing a job.
Obviously the viewpoint influences how far vet treatment will be pursued.