My stereotypical image of a transactional relationship...

We probably have somewhat different frameworks in mind and use the terms in slightly different meanings here. I don't think it's worth the time to get very precise, but there is a whiff of a definitions debate in this subthread.

because they are both getting so much out of the relationship it will be an incredibly secure one.

I don't think so, but you're already discussing it with gjm.

My stereotypical image of a "trophy wife" situation is much closer to a non-transactional one - some wealthy man is infatuated with a woman for no reason

"Infatuated with no reason" is just romantic love, often defined as "temporary insanity" :-D I think of trophy wives as a very clear transaction: the guy gets a pretty face and body, energetic sex, a symbol of high status. The girl gets lifestyle which she wouldn't be able to have on her own (or with a man of her class) and hopes for lots of money -- either as inheritance or as alimony. Personal likeability doesn't matter much as long as they don't annoy each other :-/