HRH Prince Henry of Wales, third in line of succession for the Throne (of 15 nations in the Realm) is there for more than a photo op. He’s already served as a front-line combat soldier.
The Crown is the symbol of every nation in the Realm — it’s a more important symbol for us, than our respective national flags. The Royal family has a long history of leading by example, in times of strife. Prince Henry is doing his duty, as he sees it. His presence there is worth many times the cost of some extra security. (And, if he draws out some Taliban for killing, all the better.)
What I am trying to convey is that his mere presence as a symbol of the Realm as you state makes him and those around him, including soldiers who are just trying to do their job and come home with their ass attached, a bigger target.
His presence is endangering his fellow soldiers more so than if he was not there or serving in relative anominity.
Continued photo OPs and press coverage of his deployment and location are making others vulnerable to attack.
There is no way to calculate that extra cost my friend.