I am assuming you are talking about clippers, rather than a true "shaver" like a razor blade. There are many differences in pet and human clippers. Human clippers don't have the power to get thru thick dog/cat coats. There are WAY more blades (for different lengths) for pets than there are for humans (human's generally use a short blade fitted with a guard, which will NOT go thru tangles or matting on a pet), and these blades are not interchangeable from pet to human clippers. You can find GOOD pet clippers for alot less than $300. As a general rule, the clippers you buy over the counter at pet stores, that come as a "kit" are garbage. You are better off buying a professional pair (as cheap as $130) if you are serious about home grooming.
Hairless breeds can get away with using human clippers, but for a normal-coated dog I wouln't recommend it. They may seem expensive, but consider that you'll probably save that much by not taking them to the groomer and doing it yourself.