You can purchase a keyboard with 32 keys on Amazon.com for $22.99. A keyboard this small is really best for children. Otherwise, keyboards will range from about $50 to $800 for various size keyboards, depending on the number of keys and the size of the keys.
Some keyboards don’t have “normal” size keys, rather they have “mini” size keys. More expensive keyboards will have full-sized hammer-action keys in order to provide a more authentic playing experience. A keyboard for a professional musician can cost several thousands of dollars.