I work for a company that services extinguishers. Depending on what model it is its usually worth having refilled. We charge 15.25 if its a 5 pound extinguisher, 21.95 for a 10 pounder at our shop. Most times they replace they oring too so that may bump the price up too. Also depends on how old it is. If its over 12 years old it will have to be Hydrotested on top of the refill.
Most decent extinguishers range in the 40-45 for a 5 pound new, we charge 70 bucks for a new 10 pounder. You can buy a cheap one from walmart or lowes for less but the extinguisher is basicly just a disposable, and to be frank they are known for the heads blowing off the the cylinder. You get what ya pay for.