Can anyone tell me how to clean the lenses of digital camera from inside? Every pic has a strange common spot.

and how much it costs?

If you look carefully, the spot is always in the same place; that is not the lens, is the sensor that is composed or thousands of receivers (sensitive parts), one or several are damage and no longer able to process the light that strike over them.

Not from the inside, you can't. If it's that bad, send it back or take it to a repair shop.

Are you viewing the pictures from your computer or printing them? There maybe a problem with one of those two devices.

Otherwise, if the lens is not removable you will need to get it repaired.

If you have a camera with a removable lens (D-SLR) the problem may not be with the lens, it may be with the sensor. Sensors on camera's with removable lenses should be cleaned from time to time.

You are not suppose to demantle the lense to clean it yourself, this can spoil the mechanism. I suggest you call up the manufacturer and tell them the problem, then they will guide you from there. It might not be the problem of the lense, D-SLR uses sensor or CCD to capture pictures, there might be problem with the ccd, such as condensation forming inside etc.
That being said, i have tried cleaning the lense from inside, but my camera is a conventional SLR, there are apparently some mouldy area inside, so i removed the cover (unscrew it)... and clean it. thereafter matching it back and screwing back. You are not advice to do that though.

If you use a card with the camera, it could also be the card. Sometimes the cards get dirty and they leave spots. It happened a lot at the University that I went to because everyone shared the cards and the cameras. Try cleaning the card.

sounds almost like a light leak. take it in for repair. or you can find good shops on line and send it away for repair

This same thing happened to me a couple of years ago. It was a dust spot on my sensor. They do make "clean it yourself" sensor kits, but I wasn't that brave. I sent mine back to the canon for them to clean. I have a dslr and it ran about $100. But that was cheap considering I paid around $3000 for the camera itself.