Going back through the second sons in the royal family:
Andrew served in the navy for 20+ years and since then has worked for the monarchy and nation
George VI served in the navy for 10 or so years before taking on a life of royal duties.
George V served in the navy for about 10 years before spending the next 15 years doing nothing much with the occasional royal duty before he became King.
Prince Alfred - again in the navy for about 20 years and then inherited the Dukedom of Saxe-Coburg and Gotha.
Frederick, Duke of York - served in the army until he had to step up to be Regent for his older brother in Hanover.
In fact for 2nd sons of the monarch the expectation is that they will do royal duties after 10 - 20 years in the military - exactly as Andrew has done and is doing.
The downside is you may see him in more questionable business deals, such as with the Kazakhstan people, like Prince Michael or Lord Linley
You make a good point here as to why he should remain doing royal duties actually.
Charles makes about 16 million a year from the Duchy of Cornwall and other enterprises and mostly pays his way from that, and is not on the public purse except possibly for security.
Charles supports himself, Camilla, William, Kate, George and Harry from the Duchy of Cornwall income. Of course there security is paid for by the British public - as would be the case for any person deemed 'at risk' with Harry having more than others due to threats since his service in Afghanistan.
Cornwall may be inherited but he works at it, and William is taking classes to be prepared to work at it and mostly pay his way when Charles is King.
Cornwall isn't 'inherited' as it is a different Duchy to the norm. To be The Duke you have to fulfil TWO criteria - eldest son of the monarch AND heir apparent. Being one of those doesn't make you Duke e.g. if Charles dies before The Queen, William couldn't be Duke of Cornwall as he wouldn't be the eldest son of the monarch and so the Duchy income wouldn't go to him. Of course assuming Charles becomes King William will fulfil the criteria and so will become Duke of Cornwall and expected to support himself, Kate, George, and any future siblings until he becomes King.
Charles will then have access to the Duchy of Lancaster estate which currently provides The Queen with an income of around 13 million pounds from which she supports the rest of the royal family and thus subsidises the work of the royal family on behalf of the nation.
That is a separate income from the Sovereign Grant which is simply the payment to cover expenses, maintain the royal palaces and pay the transport costs. As, in real terms, it is about a third of the level it was in 1992 there is a massive backlog in works that need to be done - but that is because the British public and government refused to maintain their heritage - so be it.
William also has his inheritance from Diana but he doesn't need to touch that as Charles is supporting him although there are rumours that he has dipped into that money to assist the Middleton's buy their new home at Bucklebury.
Harry has inherited much from Diana and may be expect to not take much from the public purse except possibly security.
Harry costs more in security due to his service in Afghanistan. His public duties will also be paid for by the taxpayers - so if he does a lot of engagements he will cost a lot but if he continues to do around 50 a year not so much.
Charles will also probably cut security for Andrew's daughters, for whom Andrew was not able to acquire public roles.
Charles won't control who gets security as that is a decision for the Metropolitan Police and Andrew's daughters lost theirs when Eugenie left university. Most of the royals only have security now when doing public duties and not when on private time. Only the children of The Queen, Camilla, William, Kate, George and Harry have it 24/7 - not even Sophie has 24/7 anymore.
There will still be free tickets to wimbledon, but you won't be paid to attend or take a helicopter there.
No royal gets paid to go to Wimbledon now and none take helicopters there. The Middleton's get as many free tickets to Wimbledon as do the York girls it seems.
The royals don't get paid any money to just be royal or to turn up to events. Their expenses in getting their are paid but they actually support themselves either from private sources or from The Queen with Charles supporting only his own immediate family.
But as it stands presently the British Royal Family costs much less than the Presidents of other countries like the US and France and it is mostly for buildings that brings in money from ticket sales. Charles thinks he can make it even more affordable.
The problem with this section is two-fold - one no one can actually point to any statement from Charles suggesting that he is going to cut expenses. It is one of those statements that was reported as coming from him by a junior staff member and has grown into a fact with no evidence at all.
If they do cut down the number of engagements - fine - but they will also be seen less, and the ordinary people will miss out.