Nikon digital SLR camera names?

What does those names mean, Nikon D3000, Nikon D7000, Nikon D90 etc., and why is D90 expensive than the D3000, and the D3 more expensive than D90 etc.

I don't think there's any meaning to those names.
Its just model numbers. If they all had the same names it get confusing.
Its like movies. You get part one and part two to serparate them.

They are model numbers.

The D3 is a professional camera, full frame, thus costs more than the cropped sensor D90 or entry level D3000.

The Nikon D7000 is th newest version of the D90 which now includes Full HD video (1080p) and can use auto-focus in the video mode.

Here is a link. You can compare the differences between cameras here.

http://www.nikonusa.com/...index.page" class='ext_link'>http://www.nikonusa.com/...index.page

Sample video using th D7000

http://www.nikonusa.com/Find-Your-Nikon/Digital-SLR/index.page

Ok, basically all Nikon products are named for their intended usage. With Film cameras any camera designed for professional use was an F series camnera, F-2, F-3, F-4 F-90, and F-5. Amateur cameras were the N series, or EM.Pro series cameras are built more rugged to meet the needs of photo journalists. The F series had more accessories available and things like interchangeable viewfinders and things like that.

With the digital cameras there's also a amateur and pro series. The digital cameras made with specs for professionals are the D-1, D-2 and D-3. Each of these models has more options, and different metering and also can shoot in different modes like RGB, and TIFF for raw editing, there's a High Speed Model that just like in film cameras has an H after the number, and there's an X model also. They also have different metering and full frame image sensors.

The reason that the D-3 is more expensive is that it has better electronics, is designed for more rugged use and is more strongly built than a D100 and has a larger CCD. Which means larger pixels, so a 2.1 MP D-1H can give an image as sharp or sharper than a 8 MP compact camera. Plus it is built to withstand all environments.

The more digits in a number denotes lower class, however higher numbers within those are better (so 7000>3000). Similar also happens with Canon (X>XX>XXX).

It means nothing. It's just used to name and differentiate products from one another. If you don't use separate names to name your products, no one will know which one is which. (this should be obvious).

Some cameras are more expensive because they are built better and have more features or newer technology. It's the same for any other products within the same brand; a BMW 7-series will cost more than a 3-series because it's bigger, more luxuriously equipped, and has a more powerful engine.