Actually, there is no "last dollar." If you're going to deal with the amount spent by the guests and the amount stolen by the assistant, you should substract the two dollars, not add it. The room costs $25, the guests spent a total of $27, and the assistant stole the $2 difference.
The manager originally took in $30, he gave $5 to the assistant, of which $3 went to the guests, and $2 went to the assistant. $30 minus $3 = $27.
Each guest spent $9, for a total of $27. The room really costs $25. The assistant stole the $2 difference. It all checks out.