Yes, there could be some other motivation. It's hard enough dealing with hidden motives of people you know well; trying to figure out the true motives of strangers you've met on the Internet is next to impossible.
I think you know how strange this is or you wouldn't have posted here.
The possible motives are almost innumerable.
If he is a professional photographer of children, and your daughter is such a beauty that the photographer is certain the combination of her looks and his talent with a camera will make his career take a new and amazing direction, that might certainly be one motive for wanting - even pressuring you - to allow him to take pictures of your daughter. However, this motive should be relatively easy to ascertain. Does he have a website with pictures of beautiful cherubs watermarked with his real legal name and his buisness.com address? Does he have a studio? (Why a basement? Is there something to hide?) Can he give you references from satisfied parents? If not, this is not likely his motive.
He may be an amateur photographer who wants to practice his hobby with a lovely model. That should also be easy to ascertain by looking at his online portfolio. Are there lots of good quality photographs? What does he specialize in: still lifes, landscapes, portraiture, architecture, basements, abandoned places, basements with children in them, what? How does your daughter fit into those categories?
In fact, looking at his portfolio might tell you a lot about this man's motives. Do be a sensibly harsh critic here: if he has an excessive number of mediocre photographs of children taken in basements, then he's not in it for the art of it.
The motive being hinted at in your question is pedophilia. One possible motive is that he wants to sexually exploit your daughter by taking pictures of her with the intent of distribution for money, or possibly only for his own use. (By the way, a parent who allows pictures to be taken for this reason risks prosecution.)
I understand money is tempting, and certainly if I were in desperate straits, I would probably think this was heaven-sent and I'd let my daughter be photographed for money. But I would, as you've stated, chaperone the session, and furthermore insist on the presence of, perhaps, a local off-duty vice squad member. If he's only interested in her as a subject of art, what can possibly be the objection? Money is money and art is art.
Finally, a person's interest in photographing your daughter in the basement doesn't necessarily translate to either good boyfriend material or good father-figure material. That has to be assessed in ways different from his interest in photography.
Did you do a criminal background check? It seems untrusting, but in this day and age... you can tell a lot about a person by doing so.
I hope this is helpful.
Internet-Facilitated Commercial Sexual Exploitation of Children: Findings From a Nationally Representative Sample of Law Enforcement Agencies in the United States