The soft power argument is the only one that gives me pause. I think the Queen, expecially, has exercised a great deal of soft power: e.g. hosting Heads of State, when they visit etc. It's the ultimate schmooze, especially when our elected Prime Minister is such a laughing stock and I think it does have a beneft to the country.
The £ argument holds less weight as I really don't think you need a current royal family to attract tourists to the history of royalty (buildings, jewels etc).
For me, the most damaging aspect of having a royal family is that is is the ultimate example of breeding automatically making you more valuable or more worthy to lead, in some respect. I think the knock on from that is the nepotism we see right across our political landscape and the fact that the vast majority of those in power all went to the same school together and all are interconnected. It has led us to a place of intense corruption, back handers, dodgy deals and people with some big jobs they are not remotely qualified to do: just because they once tossed a bottle of champers over another jolly bloke at the Bullingdon club.
I don't think we can challenge the latter until we end the former.
Just my tuppence worth.