Although you are advised to replace both simultaneously, they are two independent engine components. The serpentine belt is found outside the engine itself. It makes it more accessible for repair than the timing belt, which is found deeper inside. Their functions are disparate, with the serpentine belt delivering power to components such as the power steering pump, air conditioning, and alternator. On the other hand, the timing belt ensures synchronization of the pistons and valves during the four-stroke cycle. They differ in terms of physical appearance as well. The timing belt has many teeth on its surface complementary to the crankshaft and camshaft cogwheels. The serpentine belt lacks these features. Replacement costs of the serpentine belt are more affordable than those of the timing belt, the lowest prices being $25 and $300, respectively.