The best way to know what you can expect in maintenance is to call your local vets and ask their rates. And as farmpony mentioned plan for a few emergencies a year at a minimum of $300 per horse.Botulism, if you don't feed round bales, you won't need that one.
Be careful with this, we saw botulism last year on a horse that was on square bales, later found out a dead cat got baled up in it. Have seen this twice in 2 years. But I agree it is rare enough that you probably wouldn't vax for it.
Truthfully, what you really need is a rabies, tetnus, and I would suggest the 4 way. (is it 4 or 3?)
The 3 way/4 way designation is another tricky deal as depending on what manufacturer you're talking about they can be completely different vaccines. Best just to know what is in them and refer to them by that. But you definitely want to give rabies and tetanus.