I have a guitar and when I strum the strings it gives me a flat plain sound. I don’t know if its the strings or not. I heard that the type of wood changes the sound of the guitar. Is that why some acoustic guitars cost more than others?

Old strings will give a flat sound without a doubt. Change the strings for a noticeable difference in sound. It is definitely true that wood affects the sound of a guitar. The other answers appear correct but i would also add that the size, depth, and shape of the body profoundly affect sound and volume also. The bridge on an acoustic guitar also is very important seeing how the bridge transfers the strings vibrations to the top.

As far as price difference go hardware and adornments add a lot of unnecessary cost to a guitar. The addition of gold hardware and any type of inlay(mother of pearl,abalone,Etc…) can turn a guitar that costs several hundred dollars into a guitar that can get into the thousands.