There is a rulle, on which there is a general consent, that say that you cannot ask money for hospitality.
However there are some borderline cases when it could lead to some odd situations.
There is someone that after staying a few days for some reason wants to remain for a longer time, possibly many weeks. In some case the host would have the possibility to have a long term guest, but of course this guest after three days becomes a fish … not necessarily stinking, but using daily shower, cooking, wishing a warm room, charging its devices, overall, costing some real money to the host.
current couchers rules prohibit to ask money even in these cases.
I would see useful to set a maximum number of days [3 to 7, possibly chosen by the host] after when the hospex guest is no longer an hospex guest, so it can be asked for a contribution to house expenses and more.