The automobile assembly plant you manage has a Cobb-Douglas production function given by P = 20x^0.4y^0.6, where P is the number of automobiles it produces per year, x is the number of employees, and y is the daily operating budget (in thousands of dollars). You maintain a production level of 600 automobiles per year. 

If you currently employ 100 workers and are hiring new workers at a rate of 15 per year, how fast is your daily operating budget changing?