I have two identical yellow nylon halters for my mare, one for all the time, one for when I take her anyway I feel she should be pretty. I don't plan on replacing either until she wears down the one she has. It's been about two years and it's still going strong.
My filly came with a halter, that I'm assuming the owner had for all three babies they've had over the years, but she broke it about two months ago, so I had to get her a new one. So silly me went and spent $40 on a black leather halter that probably won't fit her for very much longer, but it's so pretty! I have a back up for her though, a rope halter.
I definitely think everyone should have at least one extra halter per horse, and you can never have too many lead ropes. =]