Can anyone enlighten me about the punchline?

Is it being satiric about a wall street investor, but why?


Nowadays, Wall Street investors have lost all their money, so the frog won't be able to pay the wizard. So the wizard will have to change him for free (= 'it's on me').
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When the wizard says the cost of the transformation will depend on what the frog used to be, he means that he will charge more to transform a high-earning person back into that state. Since the frog was a Wall Street investor, he is obviously impoverished and the wizard will be forced to perform the transformation for no fee ("it's on me").
Really, it's not enough of a joke to get or not get. Emotion: smile

Edit: Sorry Mr. M, I started typing this but was interrupted. I finished up and posted w/o checking to see if someone else had answered the question.
Thank you for your elaborations! Emotion: smile