Never mind 'strange amateur psychology', Brian: what seems to be figuring here is purely parents' ambitions for their daughters and an eye to the property market, without - as we've not been given any info on this - them finding out if it's what the girls would like, and without taking into account the possible pressure on the girl whose parents would rent on her behalf. I don't think I'd be entirely comfortable living in what would essentially be my mate's home, and having my parents cough up the rent to her parents. I'd feel a huge obligation to my own parents in a way I wouldn't feel towards them paying towards the entirely neutral Halls of Residence. At a time in my life where Bobbyjo has agreed you're supposed to be starting to row your own boat a bit, this situation could put a lot of pressure on the renting parents' kid that is not necessary. Forgive me for being strangely amateurish, but it's a very blokey sort of stance to take human, and especially young female, emotion out of every equation, and reduce a scenario to just cold numbers.
The whole thing sounds like it already got off to a lousy start, anyway, with the family situation as it is, and a lack of mutual enthusiasm. Just doing the math won't make it a successful venture.
BTW, I'm assuming that £3,000 p.a. in HoR gets everything paid bar the phone: electricity, insurance cover, the provision of furnishings, heating, and water. If the renting family is to pay £3,000 to the owning family, then they need to know that that's an all-in deal, too. Particularly insurance in case the flat burns down following one candle-lit party too many. They also need to be aware that the owning family must provide a carbon monoxide detector as well as a smoke alarm (that works), with batteries checked and replaced regularly, that any flues in the flat are free of debris, that there is an escape route from the flat in case of fire in the building, and that the door and window locks are also in working order. A few years ago, a lovely girl at Keele was lost because the rental property her parents had thought would be so much 'nicer' than HoR had blocked flues, and she died of carbon monoxide poisoning in her sleep. There's a wee bit more to this lark than just securing a good financial deal.