It doesn't cost that much, as we've established, it's a symbol of unity, and royalty, of ancient rights that we stand behind, they are ambassadors for our country that do huge good to charity. The role of the true head of state is utilised as is needed the royal family are there because we as a democracy choose to keep them, the moment we don't they are gone. There are no downsides, they are less of a symbol for genocide than the american president is.
Whether somethings costs a lot is subjective. However, we can say that the inbred royal family likely costs over £100/year with many estimates ranging from £200-300 million. In addition, this year, they cost the British economy £10 billion!
It's not a symbol of unity. It's a symbol of unity among particular groups (especially white Anglicans), but the Queen herself is a source of tension among Catholics, Irish, Quebecois (in Canada), and other groups.
They can be aiding charities without government-sanctioned godliness. However, most of them are losers, party animals, and inbred-inflicted crazies who do not contribute much to charity.
You can be a democracy, but not modern. That is what the UK is. It is a barbaric society where the majority can decide to "peasantize" the minority, much like how the US or UK were democracies even though they enslaved a portion of their populations.
There seems to be a lot of downsides!