Search for a non-professional dummy that doesn't have eye movement and (in most cases) is controlled by a pull-string. A pull-string dummy from ventriloquist dummy outlets like ThrowThings is usually pretty inexpensive. If you find you want to invest some more money later on you can buy upgrade packages and accessories for the dummy.
Go for the smaller, standard dummies sized at 24 or 26 inches and are generally pretty inexpensive. Also, look for mass produced character models such as Charlie McCarthy, Danny O. Day or Howdy Doody for a dummy that won't run away with your wallet.
Put your own dummy together from specialty dummy parts, since doing the assembly and painting yourself is not only inexpensive but also creative and personal. Look at dummy parts supply shops like Puppets and Props where you'll find all the essentials for a dummy that you can buy and cobble together.
Buy a used ventriloquist dummy since these are most likely to be the highest quality at the lowest price. Stores like Kallini Puppets offer a selection of second hand dummies that range from professional level to basic. You'll even find some custom and limited edition dummies that add some extra character to your dummy at a fairly good price.