Quite liking the series so far.
It seems the author is painting a picture where money is only another way of showing affection and commitment - the same way investing time into someone is. It's an interesting concept.
We saw Sumino is interested in Taiga even before he agreed into lending her the money. Also, she was the one who proposed being his girlfriend in exchange. If you take away the prejudices and preconceptions we have - where people accept money from others to be in a "relationship" only for the money itself - and assume the same view Sumino has about money (a view that Taiga somewhat also has - but never found himself something he thought it was worth spending on), you can see some sincerity on their relationship.
Sumino herself says - money is to be spend on what makes you feel good, on what you want. The value she puts on money is entirely focused on it being a vehicle for satisfying yourself. So, for someone to be willing to invest on her shows that person's commitment to her, shows how much that person is willing to be with her. If we follow this line of thought, it's not different from someone willing to spend time with their partner, putting up with whatever is needed. Like she says, it shows their resolve into being with her, and that is attractive to her.
Aight, overthinking of the week out of way.