The question is wrong about who makes that speech at the end. It is not the Baron, but the best man at Hughie's wedding to his fiancée Laura:
When they were married Alan Trevor was the best-man, and the Baron made a speech at the wedding-breakfast.
'Millionaire models,' remarked Alan, 'are rare enough; but, by Jove, model millionaires are rarer still!'
Alan is the artist who painted the picture of a beggar using the millionaire as the model. He has told Hughie that models make "a shilling an hour" for posing. This is obviously not something that a millionaire would find lucrative. So it would be rare to have millionaire model—someone who would find it worth the bother to pose for a picture just to earn one shilling an hour. Hence, millionaire models ... are rare.
The Baron's behavior shows him to be a generous person who also has a sense of humor and who can appreciate the good in others. Instead of being offended for being mistaken for a real beggar, he rewards Hughie for his generosity and even attends his wedding. This makes his behavior exemplary, i.e., he shows what all millionaires should be like; he models the behavior other millionaires should display. Hence he is a model millionaire. Since most millionaires are not like that, they are rarer still.
In other words, what Alan is saying is that one is less likely to find a generous, witty, and appreciative millionaire than to find a millionaire willing to pose for a picture. The Baron is two very rare things, a millionaire model and a model millionaire.