Two things, though. The dollar is (probably) worth more now than it will be a year from now.
Also, spending $100 for a passport she won't need for a year - so, effective use years = nine - breaks down to $11.11 per year. Paying $135 a year from now, making the passport worthwhile for its full ten years, breaks down to $13.50 per year. AND, as additional support for getting it now... a passport supersedes almost all other forms of identifications. For example, to prove one is a U.S. citizen when being hired by a new employer, a passport can be used alone; a driver's license can be used in combination with a raised-seal birth certificate (examples only; it's a "one from column A" or "one from column B plus one from column C"-type thing)
I'm going to disagree on the $11.11/year thing. It's not fair to count all the time that it just sits in your safe doing nothing for you. I'd say that if you use your passport three times in that ten years, at today's prices that's something like $35 per use. That's pretty expensive. On the other hand, my husband travels frequently for business, and he uses his multiple times each year; his is well worth the cost.
Also, the passport doesn't superceed all other forms of identification. It's a good item, but not the be-all, end-all. For example, I was at the DMV (ugh, on my birthday, I'd foolishly waited to get my license renewed and was stuck waiting) and I overheard a loud conversation in which a teenager was sent away because his passport WAS NOT ENOUGH to serve as ID for his driver's license (or permit or whatever else he was getting). What'd they insist upon? A social security card. Yes, I know that doesn't make sense, but it's true.
When my own daughter went to get her learner's permit recently, we checked the ID options carefully and she ended up taking along her birth certificate, her Social Security card, an official school report card, and her passport. We'd heard how fussy they are at the DMV, and we chose to take pretty much everything we had. Which one did the DMV folks not even choose to glance at? The passport. Seriously, they were more interested in the school report card.
Personally, I think the whole passport system is a mess. I'd be in favor of a national driver's license -- something that all adults could keep on their person, same as we all have a driver's license now. Easy to carry, difficult to damage. Our new licenses have several holograms embedded in them, making them more difficult for casual criminals to fake. And if everyone across America had THE SAME ID, it'd be easier for law enforcement officers to spot fakes.