Need to know about the types of cameras?

I need to kno why the nikon d5100 is costly than d3200, the 3200 has 24 mega pixels but the 5100 has only 16. Why is it like that? And also d90… It has only 12mpixels and costly than 5100. Same i noticed in canon models also. I know some functions may be different, but what? I also noticed a nikon's model with 36 mega pixel is also costly… Can anyone explain why?

You really, really, really shouldn't care about resolution these days. The resolution war is over. Anything over 12mp is good enough for virtually anything, and anything over 5mp is good enough for print. Now everything is about ISO and pixel size. The larger the pixel, the better the picture will look, especially in lower light.

I can't advise you on the specifics of the camera's that you've mentioned (I'm more of a Canon guy), but all I can tell you is that camera technology changes so fast that the difference between a camera designed this year and a camera made only a handful of years ago, is very visible. Even if the older camera has more megapixels.

And the professional Nikon D3s costs $6, 000 also has a 12 Megapixel sensor.

It is because there's more to a camera than the number of pixels. For instance, the D90 has features such as an in-body focus motor (for using AF lenses), speedlight remote commander, Wireless and Wired remote control, Auto FP High Speed Sync, better built body (magnesium frame - less plastic'ky), and so on that the entry level D3200 and D5100 cameras lack.

In fact, you generally need to double the megapixel count to see any significant difference in resolution, so there's absolutely no discernible difference between 12, 14, and 16 Megapixels.

The reason the D800 (36 megapixels) is more expensive is that it is:

1.a new camera. New cameras always go up in price.
2.professional camera - so it has features that the lower cost cameras lack.
3.it is a full frame camera with a sensor 2.3 times larger (in surface area) of a DX camera.