Smart phones cost about $500, but can be discounted with a contract. However, a data plan is required and a monthly phone bill of about $100 is standard. The real cost of owning a smart phone must be factored in before buying.
I recently bought the Motorola Droid for $550. On top of this cost, I had to sign up for a data plan of $35/month. My yearly phone bill just increased $420 by getting a smart phone.