Well actually, 20 dollars is 4 days of groceries. 20 dollars is an amount of money that me, as a working adult that hasn't got a lot of money to spend, that i just cannot spend frivolously. The base game is going to be around $40, that will have a large island, around 100 creatures, 4 tiers of buildings and a lot of other items. This DLC is half that price, with 1/10 of the creatures, 1/2 the map, 1/10 the items. So no, it's not worth the $20. They probably overshot themselves when they started giving out price money.
But compared to the base game, this expansion is not worth $20.