Current prices (June 2012) are: Rs 750 (for foreigners) for Taj Mahal (250 entrance + 500 levy) and ₹300 for Agra Fort (250 entrance + 50 levy). One gets ₹50 discount when presenting ticket for Taj Mahal at Agra Fort. The Taj Mahal entry fee also includes a 500mL bottle of water and shoe covers. Make sure you pick them up when you buy your ticket. For Indians, its Rs 20.
Official guides are available for Agra for INR 900 (approx US$ 20) for a half day (including Taj Mahal & Agra Fort). Contact your hotel for details. Any guide that charges less than that is probably an unlicensed tout. Most unlicensed touts have fake IDs and focus more on taking you shopping rather than on presenting accurate information.