You can get a basic corded phone for about $15. If you want a phone with all the bells and whistles, you should expect to pay $50 to $100.
If you choose a cordless phone, the cheaper units run about $20 and again you can pay well over $100 to get a cordless phone system.
If you are looking for a cell phone you can get one Free with new service or an upgraded contract on most carriers. If you want something nicer, you could pay as much as $200 with a contract or $500 plus without one.