It depends on the size, age and sex of the spider. If it's a guaranteed female it's going to be more expensive. If you don't care about those factors, you can get a juvenile or a baby for far less from a reputable tarantula breeder. I'm in Canada and I've bought my babies from Tarantula Canada. If you're in the US, the best thing to do is to find a breeder that's closest to you. You can find a breeder by doing a little online research, as most are available online and you can buy and they will ship to you. Additionally, you can check the buy/sell forums on arachnoboards:
I own a 5 year old red knee (female) among many other species. This species is relatively docile but sometimes is skittish or nervous. As juveniles or babies they are quite speedy but non-defensive. This is one of the species I recommend to those new to the hobby. As they mature they get less speedy and are not as prone to kick hairs (a defensive measure, they have urticating hairs on their abdomen which they kick when they are frightened).
Like most tarantulas they are easy to care for, all they require is a medium pet pal (or smaller if not an adult), a shallow water dish (with pebbles in the dish if the spider is little to prevent drowning in the water), a hide such as a fake plant or piece of wood and substrate of about 2" in the bottom of the habitat. In order to provide hydration you should lightly mist one half of the habitat every three days without over-wetting. Red Knees are accustomed to a somewhat drier environment.
My red knee is one of my favorites but she did cost more than the average or more commonly available spider. Still, as a pet they are inexpensive to maintain after the intial purchase of the spider. It only needs one or two crickets a week.
Feel free to email me if you have any further questions.
They're a little pricier than some of the more common and more-of-a-beginner's species, like rose hairs and pink-toes. The smaller of a spider you purchase (baby spider or sling - contraction of spiderling, I guess), the less expensive they are but the care is a little more delicate, too. I'm going to say $40 to start? It's been a few years since I bought any. Try the arachnoboards message board or Petco - the message board has lots of breeders selling stuff on there and Petco carries the more common stuff.
They're supposed to make a good pet - grow to a good size and fairly docile and one of the more colorful, of course.