After about living for 100 or so years after Arthur's death, Merlin realized he wasn't going to die anytime soon. So how does an immortal man plan for the future? Easy, he collects coins and jewels, that would one day be worth millions of pounds. Perhaps billions
As the centuries passed, Merlin collected more and more coins. His home was filled with them, but the time was not yet right to put their value to use. He had to wait for the right moment. If he used their value to soon or to late, all his careful saving would have been for nothing.
By the time he reached the late 1990's he was the richest man on the planet. He owned almost all of the forests in Europe, as well as a castle (to be more precise, Camelot's castle). Even after purchasing all that, he still had money.
He paid three different companies to clean the castle on a bi-monthly basis. He also paid them to keep quiet about the origin of the castle. If anyone asked, it's just a plain old castle from the Middle Ages. It is not the legandary castle from children's books.
He wanted the castle to look the same as did when Arthur last saw it.
He would have used magic to keep them quiet, but he didn't like twisting people's minds. The warlock found that using money was easier.
By the time 2010 rolled around, Merlin had several camps set up in the forests. Many of them were free camps. Camps where less fortunate people could send their kids during the summer. Every meal was healthy and filling. Activities ranged from Arts&Crafts to Paragliding and Swordplay (harmless swords).
He owned three hospitals, where people could be treated by the best doctors and nurses in the world.
He owned and help build several different schools that would help those mentally or physically challenge learn to overcome their ailments, so that they could be whoever they wanted to be.
He would continually donate money to those who needed it. He volunteered at soup kitchens every chance he had. He would offer to buy groceries for people who didn't have enough for what they got. He would even get them more groceries than what they had if they had kids.
Oddly enough, no one knew who he was. Everything he owned was bought under aliases. People who saw him and had the pleasure of having a conversation with him, didn't know that he lived alone in a one-room cabin by the lake where Arthur's final resting place is.
No one knew he was richest and most powerful man to ever walk the planet. And he preferred it that way.