How much do professional (not just like digital) cameras cost?

I'm 15 years old, and really like photography. However All i have is a battery-operated digital point and shoot camera. I'd like one of those nice black ones with a big lens, like Nikon or whatever. I'm new to the fancy camera world- where can i get a cheap one? Any recommendations?

Your starting point for a professional digital camera is around $4000. Something like a Canon EOS 5D MkII with a 24-70mm f/2.8L lens or the Nikon D700 with a 24-70mm f/2.8G ED lens.

I don't think that's what you want. Below is a list of cameras with approximate prices.

Entry level:

Nikon D3100 (18-55mm lens) --- US$550 / AU$740 / UK£440
Canon EOS 600D / Rebel T3i (18-55mm lens) --- US$900 / AU$1090 / UK£750
Pentax K-x (18-55mm lens) --- US$500 / AU$610 / UK£380
Pentax K-r (18-55mm lens) --- US$640 / AU$870 / UK£470
Sony Alpha A390 (18-55mm lens) --- US$500 / AU$560 / UK£390
Sony Alpha A55 (18-55mm lens) --- US$850 / AU$1100 / UK£600

Enthusiast / Mid-level:

Nikon D90 (18-105mm lens) --- US$960 / AU$1100 / UK£680
Nikon D7000 (18-105mm lens) --- US$1500 / AU$1500 / UK£1060
Canon EOS 60D (18-135mm lens) --- US$1300 / AU$1450 / UK£990
Pentax K-7 (18-55mm lens) --- US$980 / AU$1260 / UK£720

As you can see, prices range from $500 - $1000 for entry level cameras and from $1000 to $1500 for mid level cameras.
Of the entry level cameras I think I'd recommend Pentax as a good camera giving a lot of bang for your buck.
I think all of the mid-level cameras are good performers. I guess the D7000 and the K7 jump out at me because both have magnesium alloy bodies. But for the price the D90 is a brilliant piece of kit.

Professional digital cameras cost between $4, 000 and $42, 000. I think you are assuming that all DSLR cameras are professional cameras. This is not true. A starter DSLR costs about $500. Additional lenses cost extra. There's no place that sells them cheap.

You could buy a used DSLR and save a lot of money. B&H, Adorama, and KEH are all trusted sellers of used equipment and they all give you at least a 30 day warranty on used items.

You can also find excellent deals on used DSLRs on craigslist and ebay but you are taking a risk. If you know what to look for and are careful you can do very well.

Professional grade cameras cost from around $2000 to $50, 000