Do you know any moms of sophomore girls? If you can, ask one what sort of dress the girls wore last year & what she thought was reasonable. If the mom tells you long prom-type dresses are common, that's one thing. If she tells you either short dresses or a mix of short and long dresses are "it", then that's another.
I can see where you've already tried to get a hard budget number from her dad. It sounds like he's not sure what's right, either, and doesn't want to step wrong on this. New territory for both of you! You might want to tell him, though, what his daughter is looking at and what information you find out from other moms. Then, the two of you can come up with a pretty reasonable range. At least he won't be blind-sided and it won't cause friction between you at the end.
For what it's worth, up until 2 years ago I spent about 6 or so years helping one daughter or another with prom dresses, semi-formal dresses, dressy occasion dresses (we had several family members get married), etc. Sometimes we bought a new dress; sometimes she borrowed one. One dress was a prom dress in May & then got cut down for a wedding in August. The least expensive dress I purchased was $35 (Bargain! And she's worn it a few times, making it an even bigger bargain!). The most expensive was either $195 or $225 -- in that range, anyway. And it was really beautiful. We bought a dress from a local bridal shop, from Nordstrom's (my favorite! not more expensive than Macy's, a better, more tasteful selection and MUCH better service!) and from Marshall's.
No girl wants to stand out in a bad way but every girl wants to look beautiful and unique -- just like all her friends. :-) You're in a tough position: you're supposed to be helpful and supportive but don't have the authority of a mom.
Good luck with this! I'm sure you'll find something lovely that makes her feel beautiful, while at the same time being appropriate for the occasion and for the young lady -- and for her dad's wallet.