Apparently this very 'small' amount of money they cost us would pay for 9.560 thousand extra nurses
Only if they are very poorly paid nurses. If you attempted to pay 9,560 nurses using the Sovereign Grant they would each earn about £4,000 per year. And if we take off one third of the money because it goes on tackling the backlog in maintenance at the royal palaces (which belong to us and would still need to be maintained even if there were no royal family) it comes down even further. If we had no royal family and didn't bother maintaining the royal palaces the money could fund around 1,200 nurses.
The Royals cost over twice as much as the Irish presidency
No they don't. The Irish presidency cost £48.1M in 2013. In 2013/14 the Sovereign Grant was £36.1M. If you want to include the private income of the Queen (Duchy of Lancaster) and the Prince of Wales (Duchy of Cornwall) that adds another £31.5M taking the total to £67.6M - still well short of double the cost of the Irish presidency.
They cost us far far more than anyone knows because the royal finances are shrouded in secrecy
Have you tried looking at the annual report for the Sovereign Grant which sets out in detail how much income the royal family receives from the government and how it is spent? The accounts for the Duchy of Lancaster and the Duchy of Cornwall are both published online as well.
And on the subject of the thread, William and Kate are funded from William's earnings and the Prince of Wales' income from the Duchy of Cornwall (which comes from the land owned by the Duchy, not the government). The amount of taxpayers money going to the royal family will not change by a single penny no matter how many children William and Kate have.