This is how it works...

The "extra" $2 is subtracted, not added.

Each person pays $10 to equal $30.
The final price is actually $25. The hotel has $30. Each person is given back $1, and the guy takes $2. Thus, $30 - $5 = 25.

30 - 1 - 1 - 1 - 2 = 25

Another way to look at it:
With each person paying $9 the hotel recieves $27. The bill is only $25, so the guy pockets $2.

9 + 9 + 9 - 2 = 25

It's not even a riddle. It's a cleverly phrased question.

What is it they say about silence being golden?