The £30m figure is pure speculation, and includes the private costs which were paid by the Royal Family from their personal funds. The taxpayer paid only for security - the costs have not been published but for William and Kate's wedding were £6.35m BBC News   This is roughly the same as what it costs the Met to police football matches in London (they were able to recover only 5% of the costs) over which no one bats an eye.

The Sovereign Grant is paid for out of the profits of the Crown Estate, which in 2016/17 yielded £328.8m to the Treasury. In the same year the Sovereign Grant was £42.8m. It is due to nearly double, but that is due to the exceptional costs of carrying out essential repair work to Buckingham Palace (which belongs to the state, not to the Queen, and the work would need doing whether it is to house the head of state or turned into a hostel for the homeless). There are additional security costs in addition to the Sovereign Grant of around £100m but the total costs are still less than the overall return from the Crown Estate.

My point is that this costs the ordinary taxpayer nothing, and there is more than enough surplus from the Crown Estate income to cover the occasional exceptional cost of something like a royal wedding. That's without the tourist income the royal family attracts, estimated at around £550m. There may be very good reasons for preferring something different over a monarchy, but the cost should not be one of them.