A video-recording system was purchased 2 years ago at a cost of $36,000. A 5-year recovery period and DDB (Double Declining Balance) depreciation have been used to write off the basis. The system is to be replaced this year with a trade-in value of $5,000. What is the difference between the book value and the trade-in value? The difference between the book value and the trade-in value is ______$ .